I’ve had a Mac Pro for a year and I’ve been using mostly XP via Bootcamp during that time. Only one shortcoming that I can think of; XP will only “see” 2 GB of RAM and ignore the rest. Whether AE 8 is able to access the “invisible” RAM or not, I don’t know, as I’m still running AE 7.
Oh, and when you buy your Mac Pro, don’t even think about not buying the AppleCare protection. A month ago my Mac Pro completely died. Wouldn’t even turn on. They ended up replacing the power supply and the logic board. I didn’t lose any data, but I would have been out about $1,500 if I hadn’t had the AppleCare!