I wouldnt go above 2.5-3gb. Even 3gb isnt needed. 2gb should be enough for anything today. But, the more memory isnt always better. It is to an extent, until you get so much memory that it starts to actually decrease performance.
I heard somewhere, that there is a cpu with a memory controller that supports up to 1tb of ram or somethin like that......forgot where i heard it though.....thats alot of ram!!!