Other options...
You can also put in a GeForce 2MX TwinView card - it's cooler than the faster cards, and gives you an ADC connector and a VGA connector.
Other cards that fit the Cube are the GeForce 3 and the Radeon 7500 - I believe that the 7500 Mac Edition
does have dual monitor support, unlike the GF3 (which is better than a GF4MX in many cases - and is why they are rarely seen on eBay...). Just to be sure, though, check out
http://www.cube-zone.com for a complete list of viable cards for your new arrival...
Welcome to the Cube Club!