Windows 7 works just fine with the games and with mods. It uses RAM more intelligently than Vista did. Just heed the warnings about the UAC and installing outside of the default directory.
What works well for Oblivion hasn't changed much. The game won't use much of a second core, so the clockspeed matters more than the number of cores. VRAM is important for the mods: 1GB works well. Arthmoor has a 2GB card, but that is probably out of your price range. Don't worry about multi-card setups via SLI or Crossfire. Expensive and the boost in performance isn't commensurate with price. A good fast harddrive won't hurt, but don't waste money on an SSD (solid state drive) yet. I've been watching the prices, and they need to come down quite a bit to make it worth the while.
FO3 runs better than Oblivion. It will make good use of a dual core CPU. I don't know that it makes use of more cores.
Intel only kicks the pants of ATI at the top end of the price range. I am running an Athlon II X2 250 dual core CPU at 3.0GHz. it works great.
RAM is important for both games. I run 4GB. More could be better. Less will start to hurt you. 3GB minimum, but RAM is cheap, so get enough. I am running DDR2, because of money. DDR3 is worth it, though, if you can fit your budget.
Video cards - I think right now, you will find better bang for your buck with ATI cards. I use an HD RADEON 4870 1GB VRAM. I play both games at highest settings with many texture replacers and other visual enhancement mods. Works great and only $150 or so. With more money, you could certainly do better.
To compare, I bought my motherboard, the CPU, the 4GB RAM, a 500 GB HD 5200 rpm, the video card and a 600W power supply for a grand total of $450. I kept my old case, DVD/CD burner drive, LCD monitor 1280 X 1024.
gothemasticator