jkollman,
Sorry for the delay.
Remember that PlanetPress is actually a 32 bits application. So while 64 bits systems can make it work, PlanetPress is not a native 64 bits application, and so it won't take advantage of the 64 bits environment, and there might still be a few performances and compatibility issues.
From my personnal experience, in terms of overall performances, XP and 2003 32 bits provides the best results, and it seems like your tests point to the same direction. So if it's possible for you, I would recommend that you use either one for PlanetPress. The best might be 2003 if you plan on making a PlanetPress Workflow Server.
Regards,
Raphaël Lalonde Lefebvre