Ill re-iterate - CERTAIN games are now able to use more than 2GB RAM (which would mean having 8 instead of 4 would be much better for you) but even at that point, you still need the CPU+GPU to be able to handle that much data with ease.
WoW has a 64-bit client coming, which will be able to use more than 2GB RAM.
BF3 can use 3GB at the highest settings (i believe).
Skyrim can use 3GB if it needs to, afaik.
There may be others, but those are the ones that spring to mind.
So, having 8GB can be useful for gaming, but only if you've got the other hardware to go with it.