I installed the Windows 7 Release Candidate on a roughly 4 year old laptop yesterday primarily because I wanted to see how it would preform on older slower hardware.
Windows 7 runs surprising well on on my old HP DV4155cl with a 1.6Ghz Pentium M processor and 2GBs or RAM… Overall my first impressions of Windows 7 are favorable, I’m far more likely to migrate my production systems from Windows XP to Windows 7 than I ever was Windows Vista.