Need an opinion on GPU for a Gateway!!! :(

[Payne99]

Hey Techs! Looking for input on helping a buddy of mine with his ol' futureshop purchase. I would like to just tell him to have made it custom awhile back but he was being stubborn and got anxious purchasing this PC to play games. Now that newer and better games have come out he's having issues keeping up... (All Canadian by the way)

His current spec's that I know of are:

Win 7 home 64
System Model - DX4
GPU - ATI Radeon HD5450 512MB (DDR3)
CPU: Intel Core i3 i3-550 / 3.2 GHz
RAM: 4GB DDR
Motherboard - Gateway DX4840
HD: 1TB SATA HDD
Optic: DVD SuperMulti

Now my issue is which GPU should he grab?
-Gigabyte Radeon HD 7850 OC 975MHZ 2GB for $169.99 (Boxing day sale)
-SAPPHIRE Radeon HD 7850 2GB now $209.99
and theres even a " GeForce GTX 660 OC DirectCU 1020MHZ 2GB " for $199.99 (not sure if it can use though)
and for a PSU I grabbed was:
-CORSAIR Builder Series CX600 600W ATX12V

out of these which would be most suitable for a GPU. This is just a GPU/PSU upgrade I guess, and I am not sure if his MB will allow him to change cases.

 

[PP Mguire]

Looks like a regular mATX board sitting in a mid tower. You could easily change motherboards too if you could find a good P55 board to pick up.

I would have chose a different PSU than a CX, but it'll work. My choice would easily be the GTX660. If your buddy can loosen the budget by just 30 more bucks then get a 7870.