Only upgrading RAM and the gc will give you some more performance, but you have to bear in mind that a new gc has to be fed by a fast cpu.
I had to overclock my 3200+ from 2 Ghz to 2,4 Ghz to get my 7800 GT running the way it should do. Before I had 45 FPS in Farcry, as much as with my 6600 GT, after overclocking 75 FPS.
If you want to spent 120 € for the gc, wait a little more, and get yourself a 7800 GT or even an 7800 GTX and don´t forget about the cpu performance.
If you don´t want to wait, the 7600 GT is a good choice, unless you don´t plan do play Crytek in 1600x1200!
I don´t know much about ATI, so I really can´t give you any advice about their cards.