You're just barely in the 64bit range. Adobe cs5.1 is minimum 4 gigs. I can happily alt-tab between premiere, after effects, photoshop, and bridge. I got 1gig on the gfx card and she still struggles with 1080p footage once I get over approx 2min on the timeline. Remember though this is on a laptop so my 8gig offsets the poor performance of mobility series gfx cards.
But 16gig on a gaming rig is far from overkill especially when online gaming is usually launched from a background program/task such as your internet browse or a game manager such as steam.