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#10
The memory usage at idle is based on what programs you most frequently use. The more programs you frequently use the more RAM that is "cached" in both Windows Vista and Windows 7. Overall I say that 7 is smarter with memory caching and has a smaller footprint on memory than Vista did.
Since I mostly use IE, WMP, and Office my RAM usage sits less than 1 GB idle and I have 4 GB installed. I notice a big speedup from opening programs I frequently use compared to my roommate who uses XP with faster hardware.