Im running Fallout 3 on my Vista 64 OS with 4gb of ram. No problems at all. The biggest thing you will need is a decent graphics card for the game has a very rich environment. I picked up a BFG Nvidia GeForce 9400gt at Best Buy for $100. Hope this helps.
The 9800gt should definitely do the job. I did have to update both the video card and the game when i ran into glitches before. But after i installed the updates i was fine. I have not tried any of the DLC yet. I have been playing for a while now (level 13 character) without any problems. Hope it works out for you because Fallout 3 is an awesome game.
Careful with the 9800gt. I have one and you can run into some crashing issues with fallout 3.. If you google fallout 3 crashes and 9800gt you will find TONS of forums with people seeing crashes.
One main issue seems to be the fact that the default settings for the game with this video card are HIGH quality and that takes a crapload of ram. With that setting you can easily hog up 2gb of ram within a few seconds of walking out in the Waistlands and the game doesnt behave well if memory needs to be swapped.. The waistlands are the only problem because of the immense amount of textures loaded as well as all of the actors that are processed when you get to view a HUGE distance in high graphics settings.
Most people claim that the issues go away with 4Gb of ram but I seem to still be having some crashes. I am running XP 32bit, which can only see up to 3.25Gb so I am going to try to install xp 64 bit to get the entire 4gb to see if that helps...
Basically if you could run oblivion you can run fallout 3, fallout 3 runs flawlessly with all the eyecandy up 2xaa. on my e6750 9800GTX+ 2gb DDR2 xp system, fallout 3 runs better than oblivion does FO3 is smooth as silk, Oblivion still runs like ass (I mean its not that bad, but FO3 is noticeably smoother than Oblivion).
Most of games I had had runs on vista 64bit on a raid 0 set and yes I run with tons of problems with it - mostly the cooling systems is bad the RAM is not right and the video drivers is outdated. In 64bit environment the color is very very good and make sure you have a separate sound card to enjoy the sounds.