If its an onboard video chip I think its safe to say it is probably not the best chip in the world.
In your screen section with the resolutions there is probably all of those default display depths with all but one commented out. Try switching over to a lower depth.
like this -
#DefaultDepth 8
#DefaultDepth 16
#DefaultDepth 24
DefaultDepth 32
would turn into this -
#DefaultDepth 8
#DefaultDepth 16
DefaultDepth 24
#DefaultDepth 32
try it for each depth and if they all stay at 640x480, maybe try going back to the stock drivers from the slackware installation.