I presume:
1. You have an ATI video card
2. You downloaded the latest/greatest fglrx driver from ATI's web site
3. You double-checked your X11 config file (Xorg.conf) to make sure everything looked OK.
These are probably the first three things you should look at.
Other alternatives include:
1. Try NVidia instead of ATI ;-)
... or ...
2. Try the open source "ati" driver that comes with X11 (instead of fglrx):
Finally, please check out these links:
http://www.gentoo.org/doc/en/ati-faq.xml
http://gentoo-wiki.com/HOWTO_ATI_Drivers
<= Both good even if you don't happen to have Gentoo...
https://help.ubuntu.com/community/RadeonDriver
http://en.opensuse.org/Howto/ATI_Driver