So we did some testing here on our vista machines and found Areo uses about 70 megs of ram which is significantly different then 700 megs.
That sounds much more likely.
However, if you fresh boot the system, and monitor memory usage, you should find that the memory is being used in increasing amounts, without the user actually doing anything.
If you check later, it is very likely that you could have 700 megs or much more in use.
This is a good thing though, this is how Vista is designed to work. Ram isn't something that should be kept free. If you have 2 gigs of ram and never use more than 1 gig you are wasting your money. Computer OSes now (Linux has done it for years) load files/programs that it predicts you are going to use into the available memory when the system is idol, so that when you need those things, they are available. If you need the memory for something else, it simply frees it up.
The long and short of it, is that the memory usage in Vista really isn't something most people should be concerned about either way. As long as you have a gig or so (which most new systems should now) you'll be fine.
If people are interested, Vista actually has a really slick system that allows you to use an inexpensive flash drive as a faster hard drive of sorts. The system will load up programs into the flash drive, and then access them from there when needed. It more or less acts as a much more effective swap file.