Longhorn will have 3 sections. One is Jade which is low resources. That will probably have the system requirements of 500mhz, 256mb ram, 32mb graphics. This design will be similar to Windows 2000.
Slate, which has a bit more graphic features then Jade, will probably have about 1ghz, 500mb ram, 128mb graphics required. This design will be similar to Windows XP.
The full aero, which will probably need 2ghz, 1gb ram, 256mb graphics to run propertly. I highly doubt that the system requirements will be over 2ghz as microsoft do want people to update to it lol.