jail performance questions

[V. Jones]

I'm thinking of using jails to improve security on a server
I am setting up.  Specifically, I would like to put Apache/PHP
in a jail, but I might like to set up 2-3 different jails for
different purposes.  

I've found several examples showing how to set the jails up.  
My questions involve system requirements.  Assuming plenty of 
disk space, 1GB ram and a dual processor PIII 1.13Ghz system, 
how many jails can I run?  Would I notice a significant 
performance hit if, for example, I run three jails?