I have used magento on shared hosting, vps with hosting company, dedicated server with virtualbox, tested and tried apc, eAccelerator and more, but the best solution to speed up magento is have a dedicated machine i.e. Xeon 3000 series processor, 4Gb RAM on a high speed line in a data centre, then modify your php.ini and apache.conf to enable essential mods and you will get your speeds. Anybody tested Magento on Zend Server or with Zend Optimiser? To answer the question at the start of the thread - eAccelerator seems to be the most affective and trouble free, have never had any issues with other php mods.