I am going to get an AMD 3200+ and a 7800GT and I have heard both that the 3200+ will hold the gpu back lots and I have also heard the card will reach its full potential with that processor..
That will perform really well in gaming. But I personally think upgrading the cpu will improve the performance even more, and not the other way.
But thats just my opinion
This is beacuse the 7800 is a high end extreme card, and the 3200 is still good, but there are much better processors out there.
I also hope you have considered upgrading the RAM and psu. 1gb ddr, and 400 + psu will do nicely. This will ensure the system runs well and to a higher potential.