The XF86Config file can be found in several places. On my Debian box, it's in /etc/X11.
You might also look in /usr/X11R6.
Looking through my config file, I found the section I believe is relevent to your problem:
Notice the "VideoRam" line. That is probably where your problem lies. Yours probably says 32000, and you should change it back to 16000.
There is also a script that will configure the file for you. It's called xf86config. Just type that in on the command line, and it will run you through various questions. Have information on your peripherals and display hardware ready before you run it, and it will be nearly painless.
man xf86config will give you more info on the configuration script, and man XF86Config covers the format of the actual config file.
Here's the online manual page for the config file: