You can use any PCIe graphics card with that board. And if your 750W PSU is of good quality, the power requirement of the card shouldn't be an issue either.
If you're concerned about PCIe versions, don't be. PCIe is backward compatible.
But keep in mind, a long card will cover at least 1 and maybe 2 of your SATA headers, making it difficult to plug in a SATA cable.
Regarding your CPU:
Games vary in the ratio of GPU to CPU performance used. I have an i7-860 and some games use up to 60% of the four cores (HT disabled) and some only use as little as 15% with the GTX 680 used nearly 100% in both cases.
Based on your score, you could probably buy a GTX 680, however note:
1) most games would not score as well as a GTX 680 paired with a newer CPU, and
2) the GTX 680 would perform BETTER than a GTX 670 in some games but possibly would not in others being bottlenecked.
The bottom line is that you can buy any card you wish, though you may see as much as a 40% difference in performance in some games due to your CPU.
Again, games vary significantly in how important the CPU component is so basically I'd look at the benchmarks, decide on your budget and purchase accordingly.
Your power supply, RAM and OS should not be an issue (assuming 64-bit Vista/Windows 7).
Even an HD6850 can perform nicely in many games such as Diablo 3, though other games such as BF3 won't even be maxed out by a GTX 680.