Overall, the multitasking works well in XP. The big thing to keep in mind that XP simply takes less system resources than Vista...noticably less. I have a number of clients who run XP still and I have installed about 5 Q6600 workstations in the last few months. Most of the "replaced" workstations were either P4 3.0 machines or in one case, a PD 2.8 box. The upgrade was noticable. This is with similar ram, hard drives, etc to the old systems.
I have a friend who does a lot of video editing/rendering who is running XP Pro and there is no problem using all 4 cores
