I've been running my Mac Pro with a Radeon 2600 installed until recently (GTX 275 for Windows, usually unpowered when booting to OS X) and everything has been fine. Running Snow Leopard. However, I wanted to upgrade to a GT 120 for OpenCL, and I know that the GT 120 works with the 2008 Mac Pros, despite not being listed by Apple. The card arrived today, but I'm having problems. Option 1) GT 120 by itself (Slot 2), GTX 275 in Slot 1 unpowered - Kernel Panic on grey Apple logo screen. Option 2) GT 120 (Slot 2), 2600XT (Slot 3), GTX 275 in Slot 1 unpowered - jams on grey Apple logo screen Option 3) GT 120 (Slot 2), GTX 275 (Slot 1) - black screens Option 4) GT 120 (Slot 1) - Boots into OS X, and Share Screen reveals that there is no GPU detected So, anybody with any ideas? I know that this card works in the 2008 models, so why not mine?