I would get the HD 7750 for that PSU.
I have similar PSU like yours , But it's name is 3zone and it is made in Korea. ^^
It's only 400 watts and i run 6670 GDDR5 on it. HD 7750 is the highest GPU i can go without Upgrading my PSU :P
If you want GTX 550 ti or higher.. You better upgrade your PSU to a Quality one. Just for more headroom. Even if it's another 500watts but it has higher AMPS.
Assuming you know the mobo only has x2 PCIe x16 2.0 slots, you can install a 3.0 card into a 2.0 slot?