Hey, there's nothing wrong with a Dell, its more on what the inside that counts, such as your Core2Quad, which is still a beast today.
As far as a recommendation, I would say the Radeon 5670 or 6670 would be your best bet. They are no question the best cards you can get for under $100.
Right now there though, there is some really really good stuff on sale for Christmas, like close to $100 off certain things if you can find them. This is one of the best out there right now, I actually have one. http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=154506&CatId=3670