You can test the client/server functionality by just launching the server and the client on the same machine, and connecting the client to the localhost (IP 127.0.0.1).
You can also install a virtual machine. Have a look at http://www.virtualbox.org/. The idea about virtual machines is to install an operating system over another operating system, obtaining a virtual machine. VirtualBox is very easy to install and configure.
If you have connectivity problems with your virtual machine you can use a VPN (Virtual Private Network). An easy one to use is Hamachi. Have a look at http://www.hamachi.cc/.
I'm afraid I haven't touched my project for a month and I can't remember the specifics(!) but when I utilised my wifes laptop as a "server", I discovered a problem with my RMI implementation which was not apparent when running both client and server on the same machine.
So what I'm saying is it's definately worth testing them on seperate machines.
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