I am only seeing 1024x768 under the Screen section in the display subsections, might be why
I cant remember right off hand if you need it under the monitor as well, lemme check mine...
Nope just need the sync ranges under the monitors, but you definitly need to expand your modelines in the Screen section, actually I just noticed another difference, yours is set with virtual, however in my xorg.conf it is...
Viewport 0 0
Modes "1024x768" "800x600" blah blah blah
Things to look into at any rate