I'm pleased to announce that due to the success of the site I've just ordered a brand spankin' new Dell 600SC Server with a 2.4Ghz P4 to continue to serve out spray in a more reliable fashion.
Sometimes not much at all. But usually, there are some things in servers that are more optimized for lots of I/O traffic. Like NIC's, HDD interfaces, cache, etc. And usually, you can add more drives and cards to a server - bigger power supply.
Servers generally have RAID - Lets you constantly mirror your hard drive in case something goes wrong, or allows for better performance, however you set it up. Hard drives are so cheap, its a good idea even for pc's.