I would try fixing permissions and then boot via forced loading:
at countdown press a key, now enter the boot flag:
This is the single user mode... to fix your permissions type:
Code:
/sbin/mount -uw /
cd /System/Library/Extensions/
chmod -R 755 * && chown -R root:wheel *
cd ..
rm -Rf Extensions.mkext
reboot
This will cause a forced reboot (where it loads all of the kernel extensions instead of just using the Extensions.mkext file to boot).
If this doesn't produce wanted results, don't worry, you didn't harm your system.
try booting into a forced loaded, safe mode via the boot flag:
if you can boot up this way, I think it is just your video extensions causing a problem... simply reinstall them while in safe mode.
let us know how it fares.