I decided to get a HD6850 and purchased the Gigabyte GV-R685OC-1GD. I am guessing any HD6850 will work. But this model has the advantage of needing one power connector only. It also exhausts the cooling air out the back (through a grill in the second slot) which is a major plus in this small case. It come mildly over clocked from the factory compared to the AMD reference design. It also has an enhanced cooling set up. There are two fans on the bottom of the card and these are close to the bottom of the case and to the cables coming off the motherboard there. You need to check that the cables do not fowl the fans.
The HD6850 draws less power than other competing chip sets such as the GTX460 (and seems to sell for less). Gigabyte say you should use a 500W power supply but my research into tests for this card suggest that the Dell power supply will be fine. So far there are no problems. There is a big lift in performance. I am playing Battlefield 3 on high.