To clear up a few facts (as I have as close a setup as you need to compare).
1. The 295 GTX works just fine with Windows XP 32 bit.
2. It works just fine with my Q6600 @ 3.2GHz.
3. The amount of memory that shows up in Windows with respect to 4GB on a 32 bit OS, is variable depending on the motherboard. For me Windows reports having 3GB, not 2GB as someone posted. As an aside, I am also running Windows 7 beta 64 bit. It isn't totally bullet proof yet. I have had some gaming issues, though I expected this. I just found out that nVidia released specific beta drivers for Windows 7 today (March 3rd), so I will be giving those a try.
I don't really have any useful suggestion on your motherboard dilemma, as I don't like any nVidia chipsets (thank God that nVidia finally licensed SLI on Intel chipsets (X58)), so I personally wouldn't keep either, I would go with a P45 or X48. nVidia chipsets are notoriously hot and buggy (especially their SATA drivers).