3GHz Hyperthreaded Pentium 4
2GB PC3200 RAM
512MB ATI Radeon 1650 video card (as well as a dedicated sound card)
7200RPM SATA hard drive
With a texture setting of High, as well as all draw distances (with the exception of Grass and Specular; I'll explain in a moment) on their highest setting, Fallout 3 runs like a dream. I very, very rarely dip below 30 FPS, and that's only under extremely uncommon circumstances.
So how did I do it? I just got rid of everything that looked ugly! I disabled bloom with the in-game options and edited Fallout.ini to disable grass and that stupid shiny skin over everything. Really, look up some tweaking websites with information on Fallout.ini and you'll be amazing with the difference.
At least I was, ha!
Here's another freebie: unless you have a powerful dual- or quad-core processor, turn off all the multithreaded options in Fallout.ini (with the the exception of anything related to facegen; keep that option equal to 1). Even though I have a hyperthreaded processor, those changes increased performance and reduced crashes for me.
And always back up Fallout.ini and your saved games before attempting anything like the above! An ounce of prevention....