Windows will generally use as much RAM as it is allowed. Previous experiences for me is that without a lot of added on services and background programs, Windows 7 in general regularly uses around 1.0-1.5GB of memory (give or take). If the system does not have that much memory it unloads less used data from memory into the page file so it can use that active memory for something else. That's how an operating system can get by with less than ideal amounts of memory. Of course the trade off is performance since you're constantly unloading and reloading data to and from memory, rather than just leaving it in memory. This behavior is regardless of it's instruction set. If anything, the instruction set (x86 or x64) dictates how much memory a system can access (under normal circumstances), as well as how much memory an individual program can access.
My question is, why are you so tripped up on the actual amount of memory?
edit: I've got a system that, if you kill svchost when it's trying to call home for updates, the total system with programs running only takes about 800MB. It really all depends on how the system is set up, how much pagefile you have (And how much of it you are using), and what the system thinks it needs. There is no real "It must use x amount of memory" outside having too little.