With the frequencies you set for your monitor (H 31-50, V 50-90) , you can not get much past 1024x768. Check your monitor's specs.
To get 1600x1200 the horizontal frequency must be ~80khz or greater. (careful, don't set these higher than your monitor can handle or you could damage/destroy the monitor!)
If you have gnuplot installed you might want to look at:
which is a script to plot the available modes for a given setup.
example: ./modeplot -b 185 -h 31 50 -v 50 90
(-b 185 is just an estimate for a monitor capable of 1600x1200)
The 'DEPTH, BITS PER PIXEL, AND PITCH' section of the driver's README addresses the second part of the question. (depending on how you read it