I encountered the error (ORA-00845: MEMORY_TARGET not supported on this system) after upgrading Fedora to 12 when trying to startup my oracle database. Luckally after a bit of research the solution is pretty easy to implement.
To resolve it in the short term run (as root): mount -t tmpfs shmfs -o size=1500m /dev/shm
To resolve it permanently change the line (as root) in /etc/fstab:
tmpfs /dev/shm tmpfs defaults 0 0
to read
tmpfs /dev/shm tmpfs size=1500m 0 0