I have a setup similar to this. I have my PC and A4000 hooked up to one keyboard, monitor, and mouse. The switch is called a KVM switch (Keyboard, Video, Mouse) and many companies manufacture them. I use a simple 4 port KVM from Linksys, but they sell 2 port KVMs if you want to go cheaper.
llwrath is correct in that the easy part is the switch. You will need adapters for the mouse and keyboard. Mr Hardware makes the best adapters but they are a bit expensive. Also, be sure that the mouse adapter is compatable with KVM devices, the first one I bought was not and it took me forever to figure out why the mouse wasn't working. :-)
So, there are, in the end, 5 things that you need:
1 KVM switch.
2 KVM cables (one for the PC and one for the 1200)
1 PS2 to Amiga mouse adapter
1 PS2 to Amiga keyboard adapter
You will need a scan doubler for the A1200 but as long as you have a decent PC monitor there shouldn't be any problems.
I think it cost me about $100 when I was done, but it could probably be done for cheaper.
