I can't speak for WinXP because there is no way I am forking over another $150 to Microsoft just so they can tell me how much of my computer I can alter before I have to get a code to avoid being locked out.
But I do use Win2000 on a PIII/800, and can attest that Window resizing on it (with a 32MB RivaTNT2 nVidia card) is just about the same as OS X on a G4/400 and up. The resize is a little more responsive, but at the same time, the window "flickers" during resize, something I never see on any Mac running OS X. And OS X has much more math to do on Window resizes than Win2000 - remember EVERYTHING in OS X is anti-aliased, which adds a lot of math. Also, compare the drop shadows on both OS's Windows. OS X's looks much nicer. And a final point - Icons. OS X's Icons are 128x128, even when they are set to be smaller, the system still has to deal with the extra info each icon has, as opposed to Win2000 which uses 32x32 Icons, with support for 48x48.
Still, I wish Apple would just give us an option to disable it so people who get themselves all worked up over it can just shut it down and move on to their next complaint....