The best video card you can get for a Cube is a flashed PC GeForce 6200, hands down. It will handle all the latest eye candy in Leopard and runs cool.
It's a DIY job requiring some work, but worth it for the serious Cube user. Read up on it at the Mac Elite video card flashing forums and wiki - you can also download the appropriate hacked ROM files from there.
If you can't/won't do a flashed card, your options are more limited. The GeForce 3 is built specifically for the Cube, about as fast as a GeForce 5200 but doesn't support many features and is both expensive and rare. A good card if you can find one at a reasonable price. Otherwise you are stuck with the other OEM cards - Rage 128 or GeForce 4MX.