6600GT is pretty comprable to a X1600Pro. The 6600GT will be faster in OpenGL games, X1600 should be a hair faster in FEAR and COD2. Everywhere else they'll be about the same.
256 meg (or 512 meg) vs 128 meg on an entry level card won't make any detectable difference. 1280x1024 will be doable on both with low/medium eye candy, in which case the 6600GT may perform a tiny bit better.
Realize that in modern games at high res with eye candy you're talking sub 20 fps frame rates on cards of this caliber, so even a 10% diff in performance is 2 frames/sec. Will you notice? Probably not.
edit: oop forgot it's AGP he's talking about here.