i would stick with 6 gigs, remember its not the amount of gigs you have but the speed of ram that makes it faster in the end, more gigs means it can hold more data not transfer it faster.
i recommend 8-gig's of ram i have a few guys in our mac users group here in kent wa and they all seem to stoped at 8 gig because after that they seemed to have stability issues and one of our members lost a processor so be carefull