You might need a driver for your card. You could also try changing the xgamma settings.
# xgamma -help for more info.
# echo 100 > /proc/acpi/video/NVID/LCD/brightness where NVID would be the directory where my LCD brightness settings are located.
Your may be different. Look in the ./video directory for the appropriate name. The number 100 being somewhat arbitrary. The max here is 100 again yours may be different.