The biggest factor in your decision will be based on whether or not you go with ITX or mATX. While there are plenty of FM2 (or Intel socket 1155) itx boards out there, the AM3+ socket is a fair bit bigger and I assume this is part of the reason why there just isn't a good selection of boards in the itx format (I've seen one, and it didn't seem good).
If you want to make a super compact, and good value build, an FM2 A-10 5800K with built in graphics is great, but if you want to run a bespoke graphics card it starts to make less sense.
If you are keen to go with ITX and want to run a bespoke graphics card, I'm not sure AMD is the way to go, Intel just have the platform far better covered right now.
If you are happy to go with mATX (and there are some pretty compact chassis for this format too), there's plenty of AM3+ boards.
In this case, you are probably going to choose between the FX6300, FX8320 and FX8350. Obviously the FX6300 is a fair bit cheaper, and has 3 modules vs the 4 in the 8***. It's a great value CPU and for gaming it works fine. The FX8320 and 8350 are fundamentally identical but run at different clock speeds. If you are looking to overclock, the FX8320 should be just as good for cheaper, while the FX8350 will be better straight out of the box.
Right now most games are still dual core based, though quad (and further) support is incoming pretty rapidly. CPUs haven't advanced dramatically in the last few years though, your 975 is still a perfectly serviceable CPU so you could always set up a new build with AM3+, drop in the 975 and wait to see what Steamroller (next AMD line) brings.