Ultimately you are going to have to rewrite the conf file for X. In the meantime adding a "screen=" command to your boot parameters may help.
Try (in order)
"screen=800x600"
"screen=1024x768"
"screen=1280x1024"
http://www.knoppix.net/wiki/Cheat_Codes is a good reference
How you add these depends on your bootloader. I think lilo uses an "append" command; w/ GRUB, I believe you highlight the line for the kernel you want to use & press 'e' 2ce -- 1st to edit that stanza, 2nd to edit the line. Just add your "screen= " at the end, separated by a space. Then, I believe, <Enter> & 'b' (for boot); you can double check me @
http://www.gnu.org/software/grub/man...nu%20interface
Hope this helps, at the very least it may make you desktop more bearable while you do the necessary editing to X.