Picked up Fallout 3 two days ago. I'm still not that far into it but so far I'm really pleased with it. Like all huge cRPGs it has plenty of issues (I mainly have a lot of complaints about the UI and general item interaction in the game, a lot of which will probably be fixable with user mods in the near future) but overall I am impressed. They really translated the atmosphere and aesthetic of the old isometric games into 3D amazingly well. The world really just looks awesome, and I'm running it on the lowest settings (sadly). I've mostly been doing a ton of exploration and side quests so I can't say much about the story yet. The V.A.T.S. is an adequate replacement for the turn-based mechanic of the old games (as much as I like that mechanic, I agree it probably wouldn't have translated well to a 3D game like this).
I'm impressed so far anyway. But it'll take a few days/weeks before I can decide on just how good the game really is.
I really want to get Dead Space as well but from looking at the system specs for that I'm doubtful it will run well enough on my computer at the moment. But yes, I've heard nothing but positive things about that game and the videos I've seen of it look pretty amazing.