Looks like around 5: http://www.compdigitec.com/labs/2009/03/17/windows-7-clean-i...

7 scales down like Vista never did. I installed the beta on a 4-year-old TabletPC with .5GB RAM, and it runs as well as XP ever did. I'll probably upgrade my dev workstation when the RC is released.

I think it uses about 5GB or so - that's my guestimate, I didn't pay attention when I installed last time.