Memory Issues

I'm having some issues with plugins, I'm on a fast business server with 256m allocated in the php.ini, however according to my log file I am getting hammered with memory issues, WP Multisite asking for 400-500m of memory? Shutting off all my plugins of course stops this but all I have to do is activate one of them to start the log filling up again.

Here is a line from the log
"[23-Mar-2013 23:19:16 America/New_York] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 416650928 bytes) in Unknown on line 0"
Thanks for any tips you may have.