Yuck i agree with ampm, but in more helpful terms i would recommend a 9600GT:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130328or a 9800 GTX
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130376A word of advice - watch out for your power supply with either of these new cards. You'll have to check how many watts it is. I'd be wary of anything under 450-500 if you want to run with those new cards. Also they are large, and may take up more space than your case is capable of providing - measure out how much free space you have from the slot to your hard-drives and check it with the specifications provided. (H: 4.376 in. L: 10.5 in.)
Watch your ventilation too, I'd recommend a new fan, preferably a slot fan (
http://www.newegg.com/Product/Product.aspx?Item=N82E16835888309) that you can place below the card so as to vent heat from the chassis. The new generation of nvidia cards generate a ton of heat, but have pretty good performance to go with it.
Also: never buy a dell, ever.