When the boot process stops, press Control + Alt + F2 to see if that gives you a virtual terminal.
Login as root and use the vxconf command to get the X server configuration again. I always have better luck using "vxconf" (the first entry in the menu iirc) than "auto", and it is very easy anyway.
A command that can be useful for troubleshooting is lspci, it will list the devices in your computer including your VGA chip. If you can give us the output of
lspci | grep VGA
we could try to get a good driver for your card, and configure the server in case vxconf fails again.
EDIT: Nightflier beat me, I think both answers are complementary, so I leave this anyway