Since the memory controller is on the CPU itself there is little to gain in performance with a high end board vs a good lower end board. For what board you should get it really just depends on features. If you would like to SLI your GTX260 with another GTX260 then you will need an SLI board, and currently only MSI is making such boards that also support DDR3.
If you would be interested in crossfire (ie ditching your 260 in favor of a pair of Radeon 5870s) then you have a few more board choices as there are many boards using the 790FX chipset. If you're only interested in sticking with a single GPU for gaming then a good 770 based board would suffice.
One of the 955s is C2 stepping, and the other is C3. Basically the C3 version wasn't good enough to be labeled a 125W 965. Even so, the C3 version should have a properly calibrated thermal diode while the C2 revision is more likely to give the wacky readings AMD has had since they came out with the Brisbane. Don't know why it took them so long to fix that, guess they had other issues to worry about ^_^, but anyway go for the C3 version.
A GTX 260 is still good for now and if you get one of those SLI boards you can ad another one latter. Two 260s in SLI is still very good and will offer you more performance than even a single 5870. Yeah being on a smaller budget sucks, but hey you do what you gotta do.