Well, you can definitely get HD 7770 or GTX 550 Ti.
If you'll do decide to get HD 7770, get HIS' HD 7770 iCooler
and you'll be absolutely fine.
It's very power efficient and performs greatly (for a card of "low segment"-level) at the same time.
I'm not recommending any particular GTX 550s mostly because it's quite weaker than even the most weakest HD 7770s, while it costs almost the same (or even higher, in some cases).
So you should get it ONLY in the case if you want Nvidia's card and no other, which I personally wouldn't endorse, since that will be fanboyism, and I'm not on either of sides in this war.