ATI Radeon HD 3200

You would probably be fine with a low end graphic card if its just for the monitor.

Dell would sell you an ATI 4350 with your build--which would be sufficient for basic tasks. (they would have charged you $60 for it though instead of 30)
http://www.newegg.com/Product/ProductList.aspx?Submit=E...

Personally, I would put in the extra $10-15 to get a much better ATI 4650
http://www.newegg.com/Product/ProductList.aspx?Submit=E...

This is more than you need for just the monitor... but at this price why not be safe? (you could even bump up to the gaming-ready 4670 for $60)

A larger monitor will run better with 1 gb of ram on the card. 512mb is good for must of us at 1440x900 and below, but you probably want to spring a little extra for the memory to get the best performance on a large monitor.

*I dont know if your computer is a slimline or full tower--but if its smaller keep that in mind when purchasing.
** At this low-level card you shouldnt have to worry about your power supply.