Just as a matter of comparison here...
With my 2003 spec laptop (P4M 1.6Ghz, 256 meg RAM, 64 Meg Nvidia 4200) running Fedora Core 3, X 6.8.2
, KDE 3.4
enabled, Top tells me X is using 13% memory and 2% CPU.
Of course, the alpha channel and other nifty stuff is being pushed through the GPU but it still shows the alternatives out there.
It would be interesting to see Longhorn running on a 2003 spec machine in 2006.