CHz, once I'm assured decent speeds in Oblivion on Vista, this baby is so Vista'd. I admit, XP occasionally muffs it with determining which core should be doing what - I mean, I'd dedicate a single core to System and leave one core dedicated entirely to foreground processes (ie, games, etc) but evidently XP doesn't think to do this.
Nekko, the only instance of having to disable multiple cores for games that I've found has been in games that use the Dark Engine or some variant thereof. This means Thief, Thief 2 and System Shock 2 are the only games I've encountered with that problem.