AS richc46 said, it depends and how much you have and whats needed.
Win7s RAM footprint will be a bit different on each machine depending how much you have installed.
If you leave Superfetch untouched and running, after about a week use it will take advantage of, and use even more of, any free RAM you have.
I would not worry in the least about it. Windows 7 does an excellent job at managing memory, and will make space if more is needed for your current project all on its own.
My machine, with 8GB of RAM, uses 2 - 2.4GB of RAM at idle.
30% of 4GB is Approx 1.3GB (or little less) in use. I would say this is completley normal for idle w/4gigs.
Unused RAM is wasted RAM IMHO. Let it use at much as it wants