My rig is perfectly capable of running both New Vegas and Fallout 3 (at least it should be), but right when I open up Fallout 3 from the menu screen on lags terribly, to the point of where I can't play it at all.
The weird thing is that once I got through the main menu and attempted to start a new game, the game runs fine until pretty much I enter the point where I'm 19 (If you're familiar with the campaign at all). I pushed through it thinking it might be a bug, but once I got out to the wasteland it didn't stop one bit. I've tried everything from reinstalling .net framework, directx, reinstalling the game, verifying integrity (I bought it through Steam), as well as messing with compatibility mode, disabling desktop composition, and trying to run it in Windows XP mode (as the game wasn't designed to run on Windows 7). Also I've messed with the Fallout_default configuration. I've reinstalled 4 times, but the same thing happens every time. If anyone would like me to record the difference in Fallout 3 and New Vegas I'd be more than happy to.
My main specs: i7 2600k
8GB DDR3 RAM
Mobo Asus P8P67 Pro
Also I have tried lowering the quality of game: low medium high ultra, changed water quality etc.
If anybody can help or needs more details please let me know.
I'm sure the trouble are not in your PC since you can play NEW VEGAS with no trouble (obviously), so I believe there's something wrong with the Fallout 3 itself. I'm having trouble with FALLOUT 3 too, but a minor one, its just kinda lagging on the cut-scene but nothing else.
"I know what your problem is. I literally quit playing fallout 3 until I found out how to fix it. I have the same gpu (GTS 250) and it seems that it just can't handle the water effects in fallout 3. Try going into water settings and uncheck depth of fog and turn down the water multisampling down to low. I'll be damned if this don't fix your problem. I used to lag like hell in every single turn and now I'm lag free. Also, check out the fallout 3 studder remover mod in fallout3nexus, it makes it feel a lot smoother. Believe it or not, but those water settings I mentioned also effects areas indoors, including places like megaton and rivet city. It pretty much stops the lag everywhere. I think it's because fallout 3 loads the whole wasteland map no matter where you are in the game. Hope I've helped even though this thread is kinda old.
I would also like to note that I am using quite a bit of texture mods."