You can try booting in a single user mode to recover or use a live-cd to alter your xorg.conf file to get the problem solved..it might be that the single user mode won't work either if you've messed something up big time. Live-cd gets you out of trouble most of the time, so it's handy having one around
Hint: check that your xorg.conf values are sane. If, during the driver install, you altered or let some app alter your xorg conf, make sure the refresh rates, memory amounts etc. are correct. And if xorg.conf is correct, then you better try booting a kernel without the ati driver modules..