I have one the few million Windows 7 Beta versions installed on a margial P4 1.8 GHz, 768 mb, and it installed fine and other than pointing to drivers, the install was flawless. This, after a clean install of W7 and bringing old files from XP. I am not ready to spend $ for it yet, but it does work as well as XP with V7.5 and updater.
Just FYI