Dafydd,
you should first edit /etc/launchd.conf and then add the line "limit maxproc 512 2048" inside the file.
From the terminal: sudo vi /etc/launchd.conf
add the line, save the file and exit vi (or any other editor).
To make sure that the changes are working, run "launchctl limit" before and after editing the file.
Mihalis.
Dual G5 @ 2GHz Mac OS X (10.4.5)