We currently host tens of thousands of clients and their sites worldwide. Our latest systems run on multi-core dual processors (64bit AMD Opterons) with at least 4GB RAM.
The number of clients on a server vary for each and every machine, it all depends on load averages. We put approximately 1/3rd of what the server can handle. We do this to ensure that all client sites run as smoothly as possible.
All of our shared servers use Enterprise Class Linux operating system CentOS and come with popular software such as Perl, PHP and Ruby on Rails, and MySQL pre-installed.