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 H70w/Delta 240CFM Fan: 16GB Corsair Vengeance DDR-3 1600: 2 XFX Radeon 7950 @ 1150/1575 : Corsair AX 860 PSU: x2 WD Black 1TB HDD's: 90GB OCZ Agility 3:CL X-FI:Corsair 500R Arctic White Edition:3 22inch Asus monitors Eyefinity @ 5294x 1050(bezel comp) : Alienware Tact-X Keyboard: Naga Razer gaming mouse:Corsair Vengance 1500 Headset .