Memory usage

Does anyone find that in leopard once you've opened a few programs (pages, safari, itunes, etc) that about half your RAM is used up, even after you close the programs?

I find that when I've just started leopard that about 500MB/2GB is used, but then once I've opened some programs it's about 1500MB/2GB, then once I close them it still stays like this. The only way to free the memory up is to restart the mac. It's not really an issue at the moment but I just wondered if this normal?

My memory frees back up when I quit programs.

Try this.. http://www.apple.com/downloads/macosx/system_disk_utilities/ifreemem.html