Yes, this is possible. It is pretty bare bones but it DOES work - when you minimize WoW you remove A LOT of the overhead (mostly graphics) that slows the computer down.

Keyclone / AHK should be able to send input to them... but it will be hard to see what is going on and you lose a lot of functionality (casting AoE spells, accepting quests, etc).

Any "normal" computer with a dual core processor, ideally 7 series graphics card and 2+ gigs of memory should be more than enough to run all 5. Lower will work if you only have one active and minimize the other 4. I have not and do not intend to test this - perhaps others here can better assist?