This the method I use:
Open up Ram Disk and set all the options you want (ie window size, show all icons etc) and then select the menu Window->Snapshot All option
start a shell and type
copy ram:disk.info envarc:sys/def_ram.info
then add a line to your s:user-startup
copy envarc:sys/def_ram.info to ram:disk.info
and it should work.
I'm typing this at work and don't have my miggy to hand so I might have made a mistake. Please tell me if it doesn't work.