7790 , 7770, 7750 , should all in theory work. Assuming dell had a 4870/50 hooked up to the PC you had there.
If I were you though, I don't see why you wouldn't spend a few $ on a corsair 500W unit and pick up a 7870/50
Alternativly new AMD GPUs are supposedly dropping in the next week. So I'd wait.
I5 3570K@4~5.0ghz: Gigabyte GA-Z77X-UD3H:Corsair Hydro H110 :16GB Corsair Vengeance DDR-3 1600: x2 XFX Radeon 7950 @ 1100/1575 : Corsair AX 860 PSU: x1 WD Black 1TB HDD's: 90GB Agility 3: 960GB SSD : Creative Labs X-FI:Corsair 500R Arctic White Edition:3 22inch Monitors Eyefinity @ 5294x 1050 : Alienware Tact-X Keyboard: Razer Naga :Corsair Vengence 1500 Headset: Corsair SP2500 Speakers