In a cube you have two major constraints. Size and heat dissipation. As for size, anything physically larger than the stock video card won't fit. The killer GeForce 3Ti won't physically make it inside a cube. You can't get a card with a fan inside a cube. A very short heatsink might make it, but you need to know what you are buying before you get it as it still may not fit. Trying to run a card that requires a fan or big heatsink with insufficient cooling is only going to burn up the video card. Cards that require external power are also a problem with a cube as there aren't as many power connectors floating around in there like there would be in a tower.

Edit: I thought this topic looked familiar. I posted the list of tested video cards in another thread. You should look there for what fits and what doesn't.