As a disclaimer, I'm still a complete newbie at recording acoustic guitar. I seem to spend most of my time "in the box" messing with synths, which is a shame since acoustic guitar has been my forte since forever. So now I'm trying to incorporate it into my tunes a lot more. 
 
My only mic (apart from the one you get with ARC2) is a AKG Perception 220, a large diaphragm condenser. I think it cost me around $150 - a pretty good deal because to be honest I've been blown away by its quality. Recently I've been recording some nylon string guitar for a track. I've been pointing the mic toward the 12th fret at about 8-12 inches away, and I have to say it sounds pretty good. Crisp yet warm, and pretty detailed. It sits in the mix very well without too much tweaking. Recently I read that a small diaphragm condenser is better for classical guitar because it captures more detail. So I guess my question is, it it really worth me shelling out for a small diaphragm mic, when I'm quite happy with the results I'm getting from the Perception 220? Would I be that much impressed by the improvement? Do you think the extra detail I'd get would be lost in a mix (drums, vocal, bass etc) and that it wouldn't be worth the expense unless I was recording solo pieces? 
 
Also, I kind of wish there was a little less room in my recordings. It's not that bad but I would still much prefer a dryer track on which to add my own reverb. My room is completely untreated and it looks like it's going to stay that way (ARC2 is spoiling me!). I've seen sound absorbers that fit onto the mic like this and reduce reflections:
 
http://www.amazon.com/Microphone-Diffuser-Isolation-Absorber-Griffin/dp/B0082DAL3S
 
But they're quite pricey. Is it worth it? 
 
 

Here's the M/S tech (I like it);
 
https://www.youtube.com/watch?v=wiZFtN1xm-U
 
Give it a try? You'll be happy with the results. There's plenty of other things you can do with the bus, other parts simultaneously, capos vs. open? But give this a try. And try blending multiple tracks, and pan them, EQ, etc... This is the way to go IMO. However, there are many ways to skin the cat... This is one of the good ways.

I would be looking at ways to improve the room reflections in your recordings. I have a nice dead type space which actually works very well for acoustic guitar. You may find even with a small condenser you are still going to get a lot of room in your recordings.
 
If you can dampen down the room a little I think you will find you may get the most improvement in your guitar tracks including an improvement in detail as well. It is not that difficult or costly to do either and if you are renting there are ways of making it temporary too.
 
You can always add the space into a drier recording especially now with the plethora of great reverbs available.
 
Having speakers in a slightly live environment is also not that great either ARC or not.

And yes, my recommendations earlier considered a well conditioned space (build the pyramid) Start with the space? 
 
So I was getting more at mic technique?
 
Also, you can sit a bit left of the mics shown? Or sit a bit right of the mics (as shown) and it will automatically position you in the stereo field when you send to bus. Again, if you have not tried this tech? Please do, you'll be pleasantly surprised. 

It took me quite a while, experimenting with various configurations of mic placement before I really became half way satisfied with my recorded acoustic sound.  I still have quite a ways to go.
 
IMHO, getting a good recorded acoustic sound is probably the hardest thing I have tried to do.
 
Room matters. If it's too bright and reflective, that needs to be tamed. Simple curtains and furniture and carpet will go a long way towards making the room sound different. A totally empty room has a different "sound" as opposed to one with furniture in it.
 
Experiment with mic placement and distance. Try the ARC mic. It is, after all, a mic. It will sound different from the other ones you have, but it may actually get the desired results. Worst case, it doesn't make the acoustic sound as good as a different one. But, before buying a new mic or several new mics in search of the holy grail of acoustic guitar recording, try working on the sound of the room.
 
Be sure, as well, that you like the sound of the guitar in that room with your ears. New strings are one of the things that brings my acoustic guitars alive. So when I get ready to record something where the acoustic is going to shine, and not just be a background filler, new strings are the order of the day.

You reduce phase related issues by having the distance between the mics be 4 times or more the distance between the mics and the sound source. When you use multiple mics spread them out as best you can. Separate musicians spacially if you can.
 
SDCs are good at rejecting sound from the sides (better than large diaphragms). So when you have a bunch of musicians in the same room and you are recording them all at once, SDCs should help you get more isolation between them. Some LDCs have more isolation than others. Mics that support figure 8 patterns tend to be less directional even when put to the front. I have a LDC Baby Blue Bottle that seems much more directional than some of my AKGs.
 
Directional is good when you are trying to throw out sound from the fiddle to the left of the guitarist and the banjo to the right... but when there is only a guitar playing, using a less directional mic can get you more with one mic than the directional method.
 
Sweetwater has this link about mics
http://www.sweetwater.com/insync/studio-microphone-buying-guide/
 
Presonus has this one on mic placement
http://www.presonus.com/community/Learn/mic-placement
 
This site has an interesting 'shoot out' on small diaphragm condensers that is interesting...
http://www.soundpure.com/p/neumann-km-184-microphone-nickel-finish-/565
 
For their placement, there is one in front and one aimed down at the body from up over the right shoulder of the guitarist.

Here is a good YouTube video comparing many large and small diaphragm mics.  These are all profession grade but its interesting to hear them all recording an acoustic.
https://www.youtube.com/watch?v=04vdECGx-8k
 
Rimshot
 

Someone, (I can't remember who) posted a link recently re this very subject. There were several methods of recording acoustic guitars but the one I tried and liked was the spaced pair above and below the guitar. Depending on the panning of the two tracks you can get a really nice stereo image of the guitar being played, (i.e. the sound moves from one side to the other, and top to bottom, as the guitar is strummed). The thing about using two or more mics is the ability to create a stereo image.
I seem to be able to get more detailed highs with small diaphragms but I always have a LDC in front of the guitar when I'm recording for some of the lower tones. It just gives more options at mix time.
Agree on the previous comments regarding room treatment. However you can usually find a spot in the room somewhere that will give you the sound you're after. Use plenty of curtains/doonas/blankets/furniture to get rid of any room in your recordings. All this goes out the window of course if there is a drummer or lead guitarist in the room with you.

Cakewalk have done an excellent tutorial on acoustic guitar recording here:
 
https://www.cakewalk.com/Support/Knowledge-Base/2007013311/10-Microphone-Placement-Techniques-for-Acoustic-Guitar
 
Check out the vertical spaced pair. That is way cool and I have tried it and it sounds fabulous. It works better if the mics are matched I think (but I am sure it would work otherwise too)

Some great advice here and very interesting links, thanks. I'm learning a lot while at the same time realize I'm a little over my head...I guess the main problem for me is lack of room treatment - I'm just not in a position to install any right now. ARC2 has made a huge improvement in terms of listening, but of course that's not going to help me with recording.

Which is why I'm interested in how one of those mic isolation attachments would help. I'm not expecting any miracles, but if it made a big improvement then I'd consider it worth the money.

I'd really like to get into trying spaced pairs so maybe I'll get my ARC2 mic up and running as well over the weekend, see how that sounds. Right now what I'm recording sounds pretty good to my ears, and responded well to some EQ. But ideally like I said I'd prefer less room.

I think I need to spend a day experimenting. So far I always seem to hold off on setting up my mic until I need some guitar for a track, rather than just setting it up to try different things.