For persistent difficulties, you may need to drop down to the "lowest common denominator" graphics driver (which we call the "stub" driver).
To do this, check the "Fixed Frequency Monitor" checkbox in the OS Chooser dialog or, when you start the BeOS using the BeOS Launcher, hold down the Command key, and again check the "Fixed Frequency" option. This will force the BeOS to use the generic "stub" compatibility driver.
The downside is that this will be slower for PCI graphics cards, but there is little difference for built-in video. Either way, you will be unable to change screen depth or resolution on the BeOS side (but you can still change on the Mac OS side, and the BeOS will use those same settings when you switch to it).