The faster core clockspeed, wider memory bus and higher ROP count conspire to lift the GeForce GTX 550 Ti AMP! above its predecessor, the GeForce GTX 450.
You're probably going to be running this card at a limit of 1,680 x 1,050, in which case you will see smooth frame rates at the highest settings in the likes of Far Cry 2, Dirt 2 and Just Cause 2.
More recent DX11 titles will struggle though, and you're going to have dial back the settings to achieve figures over the all-important 30fps threshold. Importantly though, the cheaper Nvidia GeForce GTX 460 768MB produces better performance at this resolution across the board.
DirectX 11 tessellation performance
DirectX 11 gaming performance (2560x1600)
DirectX 11 gaming performance (1680x1050)
DirectX 10 gaming performance (2560x1600)
DirectX 10 gaming performance (1680x1050)