You can physically put it in, yes. Will it work at full speed? Nope. Does it require hacking and slashing? Yes. Just wait a few more months and Macvidcards and Netkas might have your back. Or at least with a GTX 680
Best is GTX580 (again, power needs) and GTX 570 which, depending on your needs, may not be THAT much of an upgrade. I'm passing on it for this reason. Not enough of a change to warrant the purchase and work. And I don't want Lion anyway. Personal issue At least not for 10-15% better performance. I need at least 30-50% before I salivate.