Memory Errors OS X

I attempted to install 8.0 RC 2 alongside 7.4.5 on my OS X box, but
initdb failed with an error about not enough shared memory.

Remembering that this was a problem for starting two postmasters at the
same time on OS X, I increased the shmmax value to 500 megabytes (I had
seen something say raising it to half the available ram would be fine),
but when I rebooted my machine neither 8.0 or 7.4.5 would start.

So I lowered it to 256 megabytes, thinking there might be an upper limit
on that kind of stuff. When I rebooted my machine, 7.4.5 starts fine,
but 8.0 still will not start alongside it.

I don't particularly need both postmasters running at the same time, but
I would like to figure out the solution to this problem.

(By the way, in the course of this I attempted to manually run /etc/rc
... there were humorous results and my computer didn't really like it:
http://www.visualdistortion.org/misc/dont_do_this.png)

Jeffrey Melloy
jmelloy(at)visualdistortion(dot)org