Also i think Source was much more related to CPU speed then GFX card, at least when it comes to shadders and stuff like this.
Only a new GFX card won't make a huge difference in your overall performance.
E.g. i got a Geforce 260GTX now and had a Radon HD2600XT before.
I cannot make out a real big difference in Source performance, only some shadders are looking somewhat different now, because i also changed from ATI to nVidea.
But since you got a kewl new quad core CPU, i think you'll be set to run everything maxed out.