It's simple on Unix systems to put php5 session files into the RAM of the server. This trick should give PHP more performance, because in the RAM the files can be processed much much faster than on the harddisk.
Session files don't need many space - lower than 1 KB per file. It's not quite much. The good thing: How to do this, is described on my blog. The bad site: It's just in german. But I think, it's not quite hard to know, what the commands do. Else use Google Translator :)