I would vote for the GTX 295, HOWEVER, you will want to research this and see which performs better in which games. They seem to trade punches depending on what title you are testing. ALSO, I've seen reviews where the 295 does better in Vista and the 4870x2 does better in XP in the same title.
The Nvidia's favor the new i7 CPU's which is what you see used in alot of the test systems used for the benchmarks. So it might be difficult to actually tell which will run better in your system.
I would tend to lean to the 295, it's new and seems to do better than the x2 in most games in Vista. Which I would think is what you are running... I hope.