your Athlon XP 2500+ is fine for now. just overclock it a little, and coupled with the 6600 GT will pull out amazing gameplay, especially graphics wise
I've seen an X800 Pro on an athlon XP 2500+, the CPU far from bottlenecked it, although it didn't get *as* high as a 3200+ would have, it still ran Doom 3, HL2 and all the 3dmark series no probs at all.
an X800 Pro doesn't get 'bottlenecked' by a 2500+, so neither will a 6600 GT. it just won't get *as* high 3dmark score
also, thre is no answer to what the best video card is right now. sure, the X850 XT and 6800 Ultra are both fast, but the general rule is:
ATI performs better in Direct3D games (like HL2)
Nvidia performs better in OpenGL games (like Doom 3)
I don't dislike ATI, however I have to say the ATI's do not get a great OpenGL performance compared to Direct3D performance. however, Nvidia gets great performance in both OpenGL and Direct3D