If your system is a prebuilt, the strongest card you'll be able to add without replacing your PSU is a HD7750. Before you think of that as weak, I'll remind you that it will blow your current HD2000 graphics out of the water, and can probably handle most if not all current games on "enjoyable" settings.
Anything stronger will require a PSU replacement. A quality modern PSU has full range active PFC (no little voltage switch), and some level of 80+ certification for efficiency. Seasonic, Antec, Corsair, XFX, and Enermax/LEPA are among the better brands. Apevia, Logisys, Raidmax, Diablotek, and Coolermaster are not. If you need to replace the PSU, is it a slim case, or a standard size? Assuming it is standard, a good 500W PSU will handle almost any single card you might buy.
sir the intel i3 550 doesnt support hd 2000 graphics because it is a nephalem (lga 1156 ) part not sandy bridge!