I installed the Windows 7 beta on my old PC (Intel P4 3GHz, 2GB memory) and it’s really responsive, it’s easily as quick as XP even with all the funky Aero stuff running.  I admit, I didn’t run up 10 huge apps and see how it handled Firefox with 10 tabs (the machine for some reason, drives the fan really really hard when the graphics card is working overtime and it’s basically too loud to use these days), but in general it’s clearly useable.