how or where can I find information on the capacity of Asterisk 1.4 conference bridging? I need to know how many people can call into an conference extension.
The server that will be running the software is a PowerEdge 2950, 3.0Ghz Intel-D, 4Gb RAM.
It depends 
How many different conferences? Is everyone interactive, or muted except a speaker? Best bet is to use sipp and load test your particular server with simulated traffic simulating to your expected scenario.