Bought an Audix i5 for recording acoustic guitar, but I have to turn the gain all the way up in order to get close to a decent level.  Thinking I need a cardioid condenser.  Any recommendations for $150 or less would be appreciated.

I get really good acoustic recordings with my AKG C200B.  Highly recommended.  Ebay has used ones in your price range.
 
*for me, in as dead a room as possible, i aim the mic at the soundhole, while it is positioned 8-10" from the neck at the 7th fret(ish).

My mike for recording is a little CAD I bought a pair on a Stupid deal for $32 
http://www.musiciansfriend.com/pro-audio/cad-cm217-condenser-mic--buy-two-and-save/271324000000000
 
I find large diaphram are too boomy so the little guys are awsome. Very natural sound. 
Live I use a SM 57 and the built in PU. 

I've had great luck with the RODE NT-1A.

SDCs usually end up being preferable on acoustic.  They have a more natural off axis response which will sound more open than an LDC, and a more detailed transient response.  Not many good choices under $150.  For $199 you can get a pair of Rode M5's.  And I'll take a stereo recording over a mono one any day of the week.  You won't get the same results as true quality condenser mics, but miles beyond a dynamic.  
 

Leadfoot
I've had great luck with the RODE NT-1A.

 
+1, really nice mic for the money.

I've found the AKG Perception 220 to be a full bodied mic good for acoustic gtr and vocals.

Yes, a small diameter mic is often used as much as a large diameter condenser.  I bought 2 Oktava Mk-012s for $100 from Guitar Center, but that was many moons ago when they blew them out.  Great, if darkish mics, which worked well in the early days of DACs which tended to be a bit harsh.  Still great mics, but they are 10x expensive now, and you have to be careful of Chinese knock offs used.
 
Although SDC are first choice, if you are forced to use one good mic, I'd get a LDC since it is more useful on more things, especially vocals (tho I've used SDCs on them too).  At the $150 level I'm not sure what I'd get.  You might get find a Blue spark on sale.  And it looks cool.
 
https://www.soundonsound.com/reviews/blue-spark
 
 

Thanks for the suggestions, still doing research.
 
Found some nice demos of the Rode M3 on YT.

I find the user reviews on Sweetwater a good source of hands on info. 
Asking on these types of forums is a very narrow overview of the world of audio. But at least you can trust us :)