I agree with hokiealumnus, Win7 does use more ram, 64 uses more than the 32 bit version.
You can how ever reduce the amount of used memory by optimizing your OS, turn off the themes, turn off the transparacny, and some other stuff.
But if you want/need those things you pay the price in the memory usage.
When I had 64 bit loaded the lowest I could get the memory usage to was 460 MB or so, with 32 bit version (to many programs wouldn't run in 64 bit) I have it down to 320 MB or so. But moving from 64 to 32 I lost 750 MB due to the 3 Gig limitation of 32 bit Operating Systems. So for me it is a trade off.
Good luck with reducing the amount of ram you are using...