Oh right, ESXi 5.0 is an OS - I was just a bit confused with how the software worked as with vmware workstation I had to host the program and my vm's on an OS. I understand now.
What hardware setup would I need to run the 15 vm's that may all be used at once and may utilise upto 2GB RAM? Would it be best to go for a Rack Server or would a Tower Server suffice (even though I'd be limited to 2 CPU's with a tower)?