ATI Radion HD 5870 - under $500. nVidia Quadro 6000 - $4700 on hp.com. Is Quadro 6000 available for anything Mac? If not, why on Earth would you choose it for this comparison?

Yes a SIMILARLY configured Z800 (or the newly released Z820 which runs circles around a Z800 - or a Mac Pro) - say, with Quadro 2000, will cost around $6.6K on hp.com. Minus a 20% discount offered right there on hp.com - $5.9K. Configuring it through HP "smartbuy" system - under $5K, for those who prefer unbiased comparisons.