"...I recommend that you have at least three PCs in
your lab. This will allow you to set up and test a wide variety of
network configurations that you cannot create with fewer machines.
For example, you can set up a firewall on one box and have an
internal LAN on the other two.
It's a good idea to dedicate one machine as your workstation.
Once set up, major configuration changes and experimentation should
not be done on this box, but rather on the other PCs. This way, you
know that you'll always have one functional machine that if all
else fails you can use to get online and look up the solution to a
fix what you've gotten yourself into. It wouldn't be a bad idea to
make this one of your faster machines, since you'll be spending
most of your time on it, and many network services, like DNS and
DHCP will run fine on slower boxes.
Since you'll be spending a lot of time in front of your main
workstation, you should get high quality peripherals. Don't get a
cheap keyboard or mouse, and above all don't scrimp on your
monitor. All of these devices are likely to outlast several PCs, so
buy the best ones you can. For example, I use a Viewsonic GS771
monitor, a Logitech FirstMouse+ wheel mouse, and the keyboard from
a Dell 386 that I bought in 1992."