I used a 550ti for a while this year. At 1080p your not going to play latest titles at 60FPS on top settings. Saying that you will get playable FPS at middling settings.
What motherboard do you have? If it supports it a Phenom II 965BE get one and add a coolermaster 212+ and overclock. That should set you back about $150 and be tangibly better than your X3 athlon.
The remaining $150 could buy a slightly better GPU though not worth the tiny gain in my opinion keep that remaining 150 wait till its saved up to 250 and then get a better GPU.