> but my main one at the moment,
is how do i figure out which of the current crop of motherboards, will support the video card i already have, and want to keep?
 
If you're referring to your GeForce 8400 :
http://www.nvidia.com/object/geforce_8400_tech_specs.html
 
* Designed for PCI Express® x16
 
Then all current board should support it, or if it's the case of mini itx board, there is at least one PCI-E x16 available.

good to know about the 8400, i just might have to keep it to be able to update the other items that are more important...
i hate having to work inside a strict budget.
makes it much harder than it needs to be.
 
still chewing on all this info, i'm still doing homework

then again, i'm not doing video editing...
 
integrated graphics might be plenty......

Hyper-threading basically the CPU's with this have a means of reconfiguring themselves so they look to the OS like they have more CPU's than they really have.  
 
So a Quad Core i7 or i5 has 4 cores or four CPU's four threads (processes)
An i7 or i5 quad core with hyper-threading has 4 cores but looks like 8 cores or eight threads (processes) to the OS.

If I remember correctly it is something Intel got when they acquired the technology from DEC and their ALPHA processor.  Which could reconfigure itself to look like different CPU's.

I would think an i5 would probably do you an i7 would just do it more efficiently probably and have a little more life expectancy because of the extra processing capability, as processing load increases.

eg, i5 something like this http://ark.intel.com/products/80817/Intel-Core-i5-4460-Processor-6M-Cache-up-to-3_40-GHz no Hyper-threading.

or i7 http://ark.intel.com/products/80806/Intel-Core-i7-4790-Processor-8M-Cache-up-to-4_00-GHz 4 core 8 threads
 
they do a variant 4790K which can turbo up to 4.4Ghz.  (I use this CPU)
 
or i7 http://ark.intel.com/products/82932/Intel-Core-i7-5820K-Processor-15M-Cache-up-to-3_60-GHz 6 cores 12 threads 
 
or i7 http://ark.intel.com/products/88195/Intel-Core-i7-6700K-Processor-8M-Cache-up-to-4_20-GHz 4 cores 8  threads latest generation of Intel CPU's
 

"I've always thought it wise to keep the video separate from the MB......?"
 
This isn't a necessity. I like to keep them separate because then if I have a problem it doesn't involve my entire motherboard, so I decided to keep things more modular. Not a necessity just the way I like to work. With the i5 the video capability is on the chip.There are plenty of video cards that support multiple monitors as mine does.
 
Choices are  good to have. Can you give some idea of how many tracks you typically record, how many plug-ins you're using in a typical mix, what programs etc.? Is your present computer straining to get the job done?  I'm guessing from what I know about you so far that you might be considered an average user and wouldn't necessarily need a power user type of setup and this means you don't need to spend as much money.
 
Maybe somewhere in the middle would be more than enough. 
 
I'm very happy with my 5820K which is a bit more powerful than the i5 , but several here use the i5 and it does everything they need it to do and then some. If you can use an i5 with no problem, then you can also look at AMD processors and motherboards as a less expensive alternative. 
 

Starise,
 
i suppose, if i take a poll of my latest album tracks...
 
one song had 65 tracks.... all audio.
 
when i arrange drum tracks in superior drummer, i leave them as stereo output while in midi...
and leave them that way until i finalize an arrangement.
then i output all the drum tracks as mono 24 bit audio, and end up with as many as 30 drum tracks.
sometimes i'll pare that down, to a typical 16 tracks.
 
i do keys in midi, but commit to audio when i'm doing final mixing... (freeze synth)
 
i'm very liberal with EQ on every track,
i usually have at least 12 sub busses, all of those with either compression, limiting, eq, or all 3 on them....
at least 2 of those will be effects busses, with reverb or delay...
 
i rarely put plugs on individual tracks, rather, i'll sum them to a buss, and let the buss do the effects,
but i do use individual track effects sends/returns for room effects....
 
 
i did this whole album, "Stay", on a core 2 processor with only 2 gigs of ram, running Win XP.
did the computer strain?
 
no, not at all.
 
the only reason i want to upgrade, is that the system is 8 years old,
i cannot update any of my superior drummer midi packs and program itself, because they quit updating for Win XP.
 
i've been painted into a corner.
otherwise, with 8 gigs more ram, i'd probably be fine. don't know if my slots will even support 8 gigs, and win XP wouldn't see it anyway, if i updated the O/S, i could probably limp along fine for a few more years.
 
 

 The way you like to work in mixing seems light on the computer to me.
 
 
I'm guessing you use the regular channel EQ so nothing particularly heavy going on in the tracks. The busses could add up with plugs on them. I think you mentioned using a lot of Waves plugs..some of those can hit the cpu harder than the native plug-ins.
 
You mix way more tracks than I suspected. I'm surprised that the old computer could do that.
 
You can probably use an i5 and be perfectly fine. If you went to an i7 you would have far more flexibility to do things on individual tracks and not need to wait until mixdown, but I think this is how you like to work.
 
An i7 like one of those mentioned by Wookie  gives you some future breathing room and you can use your video card. You wouldn't need the best of the best i7. Just an average chip. I don't have the specs in front of me right now but Wookie already got you started. There is a big difference in price when you start to get into the more powerful i7 line, but you don't need to do that unless you want to do that. A mid level motherboard/ chip should keep you in business for a long time to come.
 
I'll try to get you some more info tomorrow unless someone else beats me to it.

Here's an example of a cpu/mobo combo that's a few generations back but packs a lot of punch. You get an i7 cpu at 4.4ghz and an ASRock mobo with usb 3.0.  
http://www.portatech.com/products/product.cshtml?id=79417&o=85705
 
It isn't the latest and the greatest but it's plenty powerful enough for what you do. 
 
I guess it depends on if you want "cutting edge" or are willing to use something that works fine but isn't recent. This will still run win10 64 bit. The benefit to this is cost since hardware that's starting to get older but is still new in the box won't cost near as much.
 
Going to more recent chips and motherboards will be more expensive.
 
 

thanks starise,
i'm chewing on the possibilities now..
and if i can cough up enough dough to step up to the better decision.

Let me know if I can help further. Shoot me a PM if you want. 
 
You could go for a slightly slower cpu and still be in the 3.5ghz range, be ok and save a little more dough. I didn't add the memory and cooling or the price of a new OS.
This is micro ATX form factor which would probably fit your case but I would make sure. I don't deal with these guys but they  look decent and they install the cpu for you.There's also plenty of others out there.
Good luck!