hi trevor
im also working in a mac pro dualdual core 3,0 since 1,5 years with max and vray, i´m very satisfied, costs were lower then same configurations on a dell or hp
the main issue is to use 64bit os to have to whole power of ram. i have both windows xp and vista 64bit
apple dont support xp x64 so it would be better to use vista.
on my mac pro i have the ati card. they have driver for xp x64 and its working
on my macbook pro i have the nvidia geforce8600gt. the drivers from nvidia hompage didnt work with vista x64. so i have to use a rewritten driver.
never the less the main problem you will have if you use your mac with windows os 64bit is that there is now official support, so you have to search a lot of forums to find a solution for your problems if accrue!
i never regret switched to mac!