Open your case, at the side there should be a sticker, it may say info about its wattage and other related info as 12v amperes and stuff. If your PSU doesnt have any sticker or information. I really suggest you to buy a cheap, but solid new one. A Corsair CX430 or Antec Earthwatts 380w will do the job with any new mid range GPU.
For that resolution, I wouldnt look at an hd7770, a 7850 2Gb or GTX660 would be the minimun for playing at mid-high setings most games, some on ultra, but those are few.