You don't need to buy an expensive server or 2nd computer to run ispconfig and try it! Not sure if you are aware, but vmware have made their SERVER version FREE now!
What this enables you to do - is to run ispconfig on your existing windows machine as a virtual server!
It's fantastic for testing or even a permanent server...
Anyway - it's what I use and thought I'd let you know about it..
PS: There is even a section on the website where you can download pre-built appliances and someone has built an ispconfig appliance for you. So basically all you do is download the files and fire it up with vmware and you have an ispconfig server ready to configure and run!
Enjoy - cheers...