Various video issues can affect people who installed BeOS for Power Mac. These problems seem to be affecting many Power Mac 7600s and 8500s, and perhaps other models as well. Be sure to read all the video-related questions and answers before you try any of the solutions.
First possible solution (especially for 7600s and 8500s, it seems). Apparently, there is a bug in Apple's ASICs for the built-in video on some Power Macs, which requires that video RAM be in a particular bank for the BeOS video drivers to work (this apparently affects MkLinux as well, although the Mac OS handles it fine).
Try moving your VRAM from the current bank to the empty bank, generally from bank 2 to bank 1, and try again.
Note that Be recommends that you should do this only if you feel comfortable modifying your hardware; if not, have someone do it for you, or try one of the other solutions.