Gtx 550 ti is garbage I don't recommend it although it's from a great brand. Corsair 500 watt psu $52 dlls GTX 460 1gb v2 $140 dlls
Get those two things above and you'll be set. The gtx 460 v2 is only 5 dlls more than the gtx 550ti. You'll need a good psu to power the card so I chose the corsair psu. Dynex power supplies are garbage so that's why I recommend you toss it out.
Did not see your system's spec and I now see that it's outdated. The gtx 460 will be slowed by that cpu badly. I'd suggest you get something slower so you don't waste money for lost performance.
For your system I recommend a MSI GT440 it's 80 dlls which will work with your current psu