I took out my 7300GT, and installed the Apple supplied 4870 in the lowest slot. Hooked up one monitor on DVI (a generic samsung). The gray boot screen comes up, with the Apple logo and the spinning indicator. At a certain point, the apple logo disappears and in its place is a circle with a line through it. The spinning indicator keeps spinning. This goes on for 10 plus minutes with no sign of progress. I've rebooted several times. I did not install any software - I bought the card second hand and there was no software in the box. Plus, the install manual doesn't refer to installing any software. Any ideas? I'm going to try to unplug everything next (all firewire, USB, external SATA (I have one of those cheap adapters that makes use of the onboard hidden SATA plugs, it uses the top card slot but of course doesn't actually go in the slot). Also, maybe I will boot off the Snow Leopard disk. Oh, I have the latest snow leopard running.