The current minimum specs for Windows 7 is 1.5gz, it installed fine on my 1.2gz, but adding RAM helps compensate. Windows 7 loves RAM as much as Vista, 1.5GB minimum ideal. A faster hard drive such as 7200rpm or 5400rpm Hitachi also adds help. Keep eye on running processes (only 40-45 in default install) and auto-startup programs using Autoruns freeware. Defragmenting is again user controlled, and worth running after install. Lastly, plenty of free space on drive for page files and temp files.
On the very, very bleeding edge, success with vLite slashing Windows 7 is being reported. Head over to the vLite forums to glean what information exists yet.