That's completely wrong. Vista handles memory much better and if you have 3GB RAM in XP and 3GB RAM in Vista you'll find Vista is way faster and more responsive. Also, once nVidia and ATI get some real drivers out instead of this transitionary crap, gaming performance will be better on Vista since there is less overhead. Infact, in some games already, that use Direct3D and run on ATI cards run faster in Vista, I think Oblivion was in this category. OpenGL support is absolute garbage at the moment so don't bother playing OpenGL games (Doom3/Quake4).
Obviously with a crap PC Vista won't offer performance enchancements, only slowdowns, but if you have a pretty high end gaming rig, Vista will be faster than XP in the near future, and already is in many ways.