on macbadouille they show you how to do to get an ati 9000 pro in the cube.
9000 pro is a good card, with excellent aa abilities. Id go for her.
Where you can by that card no idea? But if you know what you are doing i d buy the pc version, and flash it to the mac version. as the pc card is much cheaper than the one for macs.
Cubeowner.com has a really nice section about which cards can go into the Cube. Some needing more modification than others.
The Radeon 9000 is a good one, because it doesn't have a fan. So you keep the Cube silent. An Apple OEM Geforce3 would also fit, but the fan on those is just to freakin' noisy.
Go to http://www.cubeowner.com/faq.html as 'Zen said, there is a list of cards and waht will fit in a Cube and if the card needs modifications (or not). ;]