i see that you are looking for AGP cards, but i don't think it will make any difference for what i am about to write:
you cannot compare the x1600pro with the 6600.
the x1600pro is more like the equaliant of a 7600 or a 6800gt, while the 6600 is more like a x800.
GeForce cards have better performance on games while ATI cards have better performance on DVD and HDTV.
i have a 6600gt myself and it runs all my games at a very good speed. it can handle lots of polygons with ease.
i also tried a x800gto on the same computer and the games ran almost twice as fast. but the card was broken so i couldn't play for long, because the computer froze (some of you might have heard that story i posted a while ago).
anyway, i would suggest you got the x1600pro
http://www.sapphiretech.com/en/products/graphics_specifications.php?gpid=133
http://www.nvidia.com/page/geforce_6600.html