It is not clear to me whether you are using Apache or IIS. I use Apache, so this only applies to that.
I have not had much luck with eaccelerator nor with zend. Both had issues with pages loading. I moved to APC, but I would occasionally get crashes (which caused Apache to need to reload -- something that took only a second, but still was showing up in the logs.)
Then I moved to xcache http://trac.lighttpd.net/xcache/ 1.02 stable and things have been working really well and are a little faster than APC. I set the cache size to 100MB -- you will want at least 50MB to be sure. This has been very stable. I tried going to the 1.1RC1 and had some problems, so am sticking with 1.02.
I use a tuned version of the XAMPP distribution which is at PHP 5.1.4, but I think the above will probably work for you.
All of this improved peformance by between 50 an 100% for page loads and decreased the burden on the processors so more users could be handled.
Good luck.
--Gary