Installed Windows 7 Ultimate x64 the other night... Bloody beautiful :-) Even better because it was free! A bit step up from Vista, utilises my video card the way it should and whatever program I use seems to just glide. Booting up in comparison to Vista has a much better improvement than I thought :-)
Installed it on an old P4 with 512mb RAM, and still runs as fast as XP, with the Aero enabled too! It's a great piece of work.