A 5770 or gtx460 768Mb will do the trick. Your PSu can handle any of them.
But depending on how much you want to invest into a new video card or at what resolution you want to play the games you could buy more cheap models like gts450 or 5750 or even 5670 witch is a weak card, at list by my standards.
The question is what resolution you want to play and how much you want to spend on a new video card.
The PSU is fine. For that resolution i recommend you buy the 5750.
You do not need more than that. If you really want you can buy the 5770 for a few games that stress a lot that card at that resolution like metro 2033.
You could buy the 5770 and in the near future do a monitor upgrade to a 1280x1024 or 1680x1050 resolution at list.
If you are planning to buy a new monitor soon then a 5770 or gtx460 would be the better choice.
Once again his PSU is pretty good. Decent brand, 500w, active PFC, 80 Plus, two 18a +12v rails. It doesn't list the max combined wattage on the +12v but it should handle and a GTX 460 just fine. Closer to 40% than double the performance however.
General_Terror if you can but the model that jyjjy give here you can have a gtx460 at the price of a 5770.
In terms of performance the 5770 speed can be reached with a small overclock of the gts450. If the price difference from 5770 to gts450 is 30$ then better buy the gts450 as a better price/performance ratio. Your PSU will be able to handle any of the cards that war ever mentioned on this thread.
This is a kind of funny thread how everybody convinced themselves that the gtx460 is a great deal.. ).(including me and it s )