Most likely the PB 170 is not able to boot from a RAM disk without the help of some storage device that holds the RAM disk contents as long as the computer is off. Also it always will need a helper disk to cold boot a system up to a point to make a RAM disk operational (i.e. boot from a System floppy and copy a backup into RAM from a non volatile storage device, like internal harddisk drive, as the RAM content is lost each time the computer is powered off).

However, suspending to sleep mode and waking up again should work well with a PowerBook using a RAM disk. A P475 could be equipped with up to 132 MB of RAM, which is enough to hold a RAM disk as large as the stock harddisk drive plus leaving enough RAM for the system to work. There was some shareware available for System 7.1 to automate the task of backing up und restoring the RAM disk contents to harddisk (look for something like ramBunctious). With enough RAM you could conveniently take advantage of a large RAM for a nice speed boost regarding file operations.