I ran Xp pro 32 and Xp pro x64, dual boot, for several months. Had zero issues!
Noticed between a 5%-8% increase in game performance. The copy expired and I never bothered purchasing Xp Pro x64. I figured I'd go with Vista 64, which hadn't been released yet!
Of coarse, I never did buy a copy of Vista 64 and currently still run Xp pro 32
However, now that things have improved immensely for 64 bit support, I may revisit the idea with my next build.