I have to agree with everything NickNR says about Fallout 3 with one exception: I've been running it in Vista X64 for weeks without any problem. Perhaps I'm just lucky; my PC is a couple of years old, so it's not near the top requirement for this game.
I've spent endless hours wandering the beautifully rendered post-apocalyptic landscape, while either attempting to complete one of the many quests or just exploring for fun. The thing I really like about this game is the way you get a sense of freedom about where you go and what you do. I know Fallout 3 has an ending, but I don't feel I'll reach it soon - and I hope I don't.
I'm even enjoying the 1940's music soundtrack and find myself going around humming "Bongo, bongo, bongo"...!!
If you enjoy a good single player game with lots of atmosphere- this is for you.