New Graphics Card

Hello gang,

Probably not quite the right area for this, but I'm looking for some PC advice, specifically for gaming so I hoped someone would be able to point me in the right direction.

I recently (6 months ago) got a HP desktop of a friend who was leaving the country. Its pretty well specced, and can play most anything on decent settings. However with certain games (Batman AC, GW2 etc), pushing the settings to max doesn't quite have a good enough frame rate. I'm not too hot on PC hardware, but I think I can get top tear gaming performance with just a new graphics card (specs below). If I was to upgrade to say, a GTX 660ti or something similar, how much of a real difference would it make (without upgrading any other components)?

Core i7-2600 @3.40ghz

8gb RAM

GeForce GT 545

Any/ all advice appreciated!