I recently discarded my AMD Athlon 64 3000+, Abit socket 754 motherboard and Geforce 6600 GT 256MB for a PCI Express layout.
I didn`t go crazy, I just got an AMD Athlon 64 3800+, Abit KN8 SLI socket 939 and a XFX 7600 GS 256MB. (The 1,5 giga RAM followed me over)
The difference is really remarkable. I now have 100-130 fps in open seas with GW, and around 50 fps when recording with fraps. In addition loading times are nearly halved.
This is not a costly setup, so I recommend it to anyone.