With the exception for certain mods for Civilization 4 (out of memory errors with 6GB memory; priceless
), you don't need more than 4GB. Heck, in XP, 2GB is still plenty.
I remember when Tom's did the 2GB, 3GB, and 4GB DDR2 comparisions a while back (forgot the OS used though...). After 2GB, there were no performance increases.