Best GPU for Fallout 3: 9800 GTX+ 1GB or 4850 1GB?

Both will be more than enough at maxing out Fallout 3. However, the GPUs won't use all that RAM unless you are playing at higher resolutions or SLIing.

If your mobo doesn't have SLi/Xfire, I suggest you get a single 4870 512mb or 1GB (cheapest one on newegg now is $140 after rebate)

It performs at the level of a GTX260

Even higher than 1080P? To get the full 1GB, would I have to go 1920 x 1200?

I have roughly identical specs to your current build but I have a 9800 GTX. I run everything at 1080P with max settings without any problems. However, there are some incredible mods that have come out to increase the texture resolutions of the landscape and character models that make the game look simply incredible. I would suggest going with the 4870 as well to add those mods and have smooth gameplay.

Here is the link to site with all the mods: http://www.fallout3nexus.com/

Click on the "Top 50" and you won't regret it.

@ Shadow; At 1920x1080 you`ll need 1Gb of video RAM, if not now, than in the near future. It will not give much in the terms of framerates but it will smooth out gameplay by eliminating those `glitches` when turning quickly or entering a new location, when the system would need to load extra data into 512Mb card.
If you can, step up a leval as well-GTX260 or HD4870 would be a better choice at that resoloution.
The other cards you`ve listed are about the same in performance so I`d look more to warranty and bundle as a basis for the purchase. EVGA, XFX and BFG have the best warranty and custo9mer support.