I would just like to share my success with you all. A few weeks ago I picked up an ATi XFX HD4870 (1GB) from Ebay to install into my 2006 (1,1) Mac Pro. It was a willy-nilly purchase as I had no idea it would work correctly. To my surprise, Mac OS X 10.5.7 operates perfectly out of the box, and even displays the card correctly in System Profiler. Windows XP and 7 both work great, but I had a minor issue with output (see below). Bottom line, THE ATI/XFX RADEON HD4870 (1GB) WILL WORK IN A 2006 MAC PRO. Boot Camp displays too. So if you decide to do this, here's a few tips for some installation "bugs" I ran into. 1) Make sure you have two 6-pin-to-6-pin power cables (came bundled with mine). 2) There is very little room underneath the card if you choose to insert it into the bottom PCIe slot. It fits, but barely (see pictures). It took me five or ten minutes alone just trying to wiggle the card into the slot. I do not recommend installing it into the top slot for airflow reasons (see #5). Also, you want to remove both hard drive 1 and 2 temporarily for installation because they are a pain to work around getting the plugs onto the motherboard. 3) The two 6-pin power outlets on the motherboard supply more than enough power to run this thing. In fact, I have her overclocked in the CCC as we speak so there's no need for an external power supply like what I've seen in other video card upgrade threads. 4) One of the 6-pin connectors on the motherboard was missing the plastic cap (see pictures). If yours is also missing, it's OK, just make sure that the 6-pin cable plugs into the pins enough that it won't bump out of place. 5) This card is HOT HOT HOT. Out of the box she was feverishly idling at 90C. Make sure you get ATi drivers with the Catalyst Control Center, and bump up the fan speed. 6) The output issue had me stumped for a little bit. OS X would output the DVI through #1 port on the card, but Windows would output through #2. So if I wanted to boot into the other OS I'd have to swap the DVI plug and it became a huge pain. To fix this, I enabled a second display in Windows, swapped the cable over, and made that second display my primary and deleted the old one. So now it outputs port #1 in both OS X and Windows. And here's some pictures for your viewing pleasure: Overview #1: http://i25.tinypic.com/2ho93r6.jpg Overview #2: http://i30.tinypic.com/r0rryp.jpg Connectors on the card: http://i27.tinypic.com/16jhfg5.jpg Cables from card to mobo: http://i32.tinypic.com/a3143p.jpg Very little clearance: http://i26.tinypic.com/1zy8zgo.jpg Commandeering two slots: http://i31.tinypic.com/28ugs4w.jpg Missing power cover (plug on the bottom): http://i27.tinypic.com/zvwlz6.jpg Thank you for taking a look. If you have questions feel free to ask.