There are a few cards that you can use and it would depend on your budget and your preference for Nvidia or AMD. From AMD you would have the new release cards 7770 , 7850 and 7870 and then the older cards which are now a little cheaper , 6850 , 6870 and 6950. From Nvidia you have the GTX 550Ti , GTX560 , GTX560Ti and the soon to be released mainstream cards. It will depend on your budget and also not going over the 500w psu that you have.
I'm seeing some charts and the 7770 is weaker than a 6850 and even a gtx 468 762 mb. This card can really handle gaming with full details in 768p?
The gap between the 7770 and the 7850 is big, in performance and price, there is something between both?
I almost bought a 6870 or a gtx 560 (non ti), this cards can handle easily my system?
I live in Brazil, prices here are very high! The 7850 have the highest price that I want spend.
My PSU is a Sentey, I remember the wattage and the max 12v but not the model or other details (I'm not in home by now)
Before buying a video card you do want to look at the spec's of your power supply to make sure it will handle it. Both the 560 and 6870 are good cards and should work fine for you provided your psu is good enough. If you were to get the 7850 then that would be the best card of the group and be all the card that you need.
Best answer selected by joao_uesb.
Good luck to you with buying your new card and don't forget to make sure your psu is good enough.
Even an HD 7770 can handle your resolution pretty easily. Maybe without Anti Aliasing in some games.