Re: 7664 and 1600x1200 (maximum pixel clock?)
You might be able to get your setup to work with DVI and 7664 if you design a modeline that is 1600x1200 and still <155MHz. Assuming your monitor will sync to it, that is. You might want to give it a shot.
By the by, which card do you have that reports 155MHz with 7664? (I'm trying to confirm that the numbers are indeed a function of the card model...)
I have FX5700 Ultra -> 150MHz.