Your graphics card is substantially more powerful than mine and I can play just about every modern game without taking an FPS hit (except The Witcher 2, but that still looks gorgeous on lower settings).Glass Joe the Champ said:It's not a gaming computer in that I didn't buy it specifically for gaming, and that it probably couldn't play any current or future games like Skyrim without taking an fps hit.
Seriously, you have one hell of a gaming rig there.
To answer your question... Fallout 3 is the more compelling game, but be prepared to Google fixes to the myriad issues you'll have running it on a modern OS.
Still, I much prefer New Vegas, which also has the benefit of running just fine in Windows 7.