Something in the range of an old nVidia 9800GT would be just fine. You could even go a little bit higher than that without any major bottlenecks.
More important than the CPU are your power supply, the software you wish to run, and what quality and resolution you intend to play at.
Keep in mind also that you can always get a much better GPU which will be bottlenecked by your current system, and then keep it when you eventually upgrade your CPU/mobo/Ram.
All PCI Express cards will work in all PCI Express motherboards regardless of revision (provided you have the power supply to back it up). They will just run at the interface speed of the slowest device.