Connectix has announced plans to port RAM Doubler to the Power Macs sometime in the next few months. I don't believe the current version works on the Power Macs under emulation mode, which isn't too surprising given the low level at which it hooks into the operating system.
Connectix -- 800/950-5880 -- 415/571-5100 -- 415/571-5195 (fax) -- connectix@aol.com