Here's how I put a GeForce 6200 with a Zalman GPU fan into my Cube:
http://www.cubeowner.com/forums/index.php?showtopic=12882&hl=
Note I have a PowerLogix case, not the stock Cube case. The Zalman fan barely fits and would never go inside a stock case but there are other smaller fans that do fit the stock Cube case.