I would get a new GPU, while a new CPU would certainly give you a boost I think you're better off with a brand new GPU even if GTA is more CPU intensive. I'd go with a Nvidia card, because in my experience switching from Nvidia to ATI has caused as significant performance decrease.
Really? I had a 9800 GT 1GB and I ran the game at 1920x1080 at almost all high settings with no problems...Granted that was on an older patch but I had loads better performance than the 5670 I have now, which I expected but I can't even enjoy GTA IV now with this card whereas GTA IV was smooth as butter with my 9800.