You can use 512 MB modules, as long as they are the older 64 Megabit technology parts. These are exceedingly hard to find, and ridiculously expensive. (Most "PC100/PC133" 512 MB modules use newer 128 Megabit chips, which the B&W would only see half the capacity of.)

Note, however, that 1.5 GB is a chipset limitation, regardless of actual amount installed.