No matter which card you go with you still will face the problem with the lack of good air flow unless you mod the case somehow. Dell cases just like other mini tower types are intended for what they come with not beefing things up any for that reason.
Some have taken everything out of a Dell or Gateway, HP case to fit into a gaming style case simply to boost air flow as well as seeing a larger supply go in when wanting to move away from onboard to a good card. That's another thought you may want to look at some time.