Do you have 8GB of RAM?
On the assumption you don't (apologies if you have) -
In XP task manager it shows the overall page-file usage, which incorporates the RAM and an amount of hard drive space, used for temporary storage. I assume resource monitor is a Windows 7 move towards Apple's activity monitor, but still shows the same thing. The first 4 'reserved' GB will probably be a mostly HD based portion used for low demand system functions.
I remember in Vista, it actually showed you your real RAM usage. I have never used 7 so I'm unsure what is shown in task manager (or the 7 equivalent).