I dont really like any of those. Let me explain:
Normally, ATi cards consume a bit more energy than nVidia, and because of this, they run hotter. ( and are slightly faster in some games).
HP and DELL are not particulry known for good internal layout, decent cooling, or even decent thermal paste, so it might be an issue very fast.
I suggest if you cant test them before buying (at a store), make sure your warrnty is NOT void by changing the thermal paste for something fresh and effecive.