I just bought an XFX Radeon 7750 for use in my newly acquired 3,1 on 10.8.3 for a low cost and low power distributed computing crunching (Einstein@home). I popped it into the second slot and it worked immediately upon booting up. It's now crunching away with no problems. The XFX is a double slot card and the 7750 is bus powered. I've kept the original HD 2600 XT for boot camp access and the sweet apple logo. If anyone is interested of course. Not as exciting as a Titan or even a 7990 (with external PSU of course). But hey, figured I'd contribute a little.