My vote is for GPU. Seeing as you have an X1900 and you're looking to have a more next gen game friendly system, this is your best upgrade path. Your other specs aren't bad and certainly shouldn't be holding you back.
Yes your RAM and CPU could be better, but they are more than adequate for running todays newest games as long as they are coupled with a superior video card.
I'd be looking at 8800's or 2900's, get yourself in the DX10 game. You'll notice an instant major improvement over what you're running now. Changing out your other parts are going to show almost insignificant increases in gamability, in fact you would probably only notice the difference in 3dMark.
Asus Sabertooth 990FX | AMD Phenom II X6 1090T | 16gb G.Skill Ripjaws X DDR3 | Thermalright Silver Arrow
2 x Sapphire HD 6870 1gb CrossfireX | LG 10X Blu-Ray Rewriter | PCP&C Silencer 750 Quad | Fractal Design R3 Case
60Gb Patriot Pyro SATA III SSD | 1Tb WD Black | 3 x 2Tb WD Green | 2 x 360Gb Seagate Barracuda