Get the fan kit for the Cube. I believe Macsales sells it still and its fairly cheap and straight forward. The Cube was designed for a fan and everything is there, including a mounting bracket with holes for screws and a connection for it on the VRM board. Apple just decided not to install a fan. I ran a 7500 in my 450mhz Cube with no additional cooling and while the GPU did run about as hot as hell, it chugged along just fine.

The cube I have now also runs with no fan, but uses the stock graphics card. It had an uptime of over 100 days before the power went out recently. It doesn't have much load on it, but it does still get hot due to where the machine is physically located but it has 0 issues. It runs Debian Linux.

http://ant.ro.lt/phpsysinfo/

edit: here ya go, http://eshop.macsales.com/item/Other%20World%20Computing/FANNCBKIT/