the gf4 will be faster in general, and will be so much better if you want to use FSAA.
I would go with the gf4 personally. Hell, I already did. I had a gf3ti200 (clocked at ti500 speeds) and upgraded to a ti4200 and was very happy.
<font color=red> -- </font color=red><font color=blue>A bush in the hand is worth two birds at the bar</font color=blue>