Awesome. Very similar specs to you, only I'll be on a desktop and using an NVIDIA 295 SLI. I've seen NVIDIA issues but none so far for ATI. I wish I had self-restraint, but I'm probably buying this svcker tonight.

Uh, before you go buy...
I have two gaming systems, a laptop and a desktop. The laptop is a Core I7, 8 gigs DDR3, ATI 4680, and can run fallout really well, with no issues.
The other system I have, is a dual-quad Nehalem Xeon, 12 gigs DDR3 triple channel, and... a GTX 295. The 295 system can actually play New Vegas absolutely flawlessly, with no issues, crashes, and at ultra settings. BUT, it can only do this in XP x64. In windows 7, it has the wierd skipping music bug, lots of lag, chunks, no crashes though, but it's pretty terrible to play. So, essentially, I have to dual boot in order to get the game playable for my scenario. The ONLY things I have to run out of XP x64 are... wait for it... Fallout 3 and New Vegas.
So, my recommendation for you, would be to have a dual boot setup (personally, I use the bios "choose boot drive" to actually accomplish it, and the XP x64 sits on an eSata drive) with an XP. As I've said, I'm using XP x64, so if you don't want to go down to 32-bit, the x64 is actually an option (XP x64 is not based on the XP kernel, but the 2003 server kernel, so it's a bit different). But you will want to be running on XP.