I'm running Windows 7 Ultimate 64 bit and FO3 runs great right out of the box. Having 12 GB of RAM and a high end video card doesn't hurt either. Make sure you get the latest drivers for your video card before you install FO3.
I've had three different pc's with win 7 pro 64 bit and fallout 3 worked fine on them all, including a 4.1 ghz i7 with 16gb of ram and a nvidia Gtx 690 card, a 3.2 ghz i7 w/16 gb ram and an ati Radeon hd6950, and a 2.4 ghz core 2 duo with 4 gb memory and three different card video cards over it's lifetime. That being said there are a couple of minor ini file changes like the limiting the max AI threads to 2 and a couple to force video settings if you certain issues. Adding unofficial modes like those from fallout 3 nexus and other sites can drastically alter it's behavior. I've had my install go from running perfectly, to crashing every 15 minutes to not even starting and back again in the same day by installing and removing the wrong mods.