Yeah, the graphics card makes a difference up to a point, but the processor and the gfx card (and memory) go hand in hand, if you put an 880GTX card in with a slow processor you won't get the best out of the card because it can't process stuff as fast as the card needs.

On many cards today you can adjust the clock speeds of the gfx card as well as overclocking the processor, the key thing is heat... the faster you go the hotter things get and you need to get rid of the heat to keep your system stable.

Matt