I have a Dual Core 2 Quad 3.0 ghz processor with 4 gigs of OCZ Reaper RAM and the "CPU Usage" gadget tool on the side says I'm constantly using 30% of my RAM... that's roughly 1 gig. I have no programs running.
That's how much RAM the Vista 64 OS uses on a 4 GB system, but it isn't an issue. Read http://forums.extremeoverclocking.com/showpost.php?p=3177138&postcount=6 and if you're really concerned by the fact that you only have 3 GB of free memory, then stop and disable the Superfetch service and any other service that you don't need.