I downloaded FO3GOTY from Steam.
I have an intel i3 with Windows7 and a nvidia(geforce with CUDA)
Have had no problems running any other games on this PC.
I run the game, it loads my saves, then it freezes after about 5 mins of gameplay. What do I do?

You have a CPU with more then 2 threads, try the quadcore fix.
Open up the .ini file located in My Documents>My Games>Fallout 3. *Do not alter the one in the actual game directory labeled Fallout_Default_ini*
In the FALLOUT.ini find the line

bUseThreadedAI=0
change the 0 to a 1

then below it add the line

iNumHWThreads=2

okay, that worked. it's running smoothly so far. will notify if that changes. thanks so much!