Maximum PC

I'm a bit overwhelmed by the ways I can purchase Windows 8, and I am not sure what I need.
I want to install it, at some point, on 4 computers.
1 desktop running Win7
1 desktop running Vista
1 Laptop running Win7
1 Desktop that I haven't built yet

I would like to do a clean install on all the computers (it's still a good idea to do clean installs, right?) and I'd like to be able to wipe the drives if need be at some point and install 8 without having to install 7 or vista again (i do have the discs to prove ownership, though)

What is the best way to get this done? I'd like to buy now. I might not install anything for a few months, but the price is good now and windows media is free til christmas, so I want to jump on that.

Thanks!

I think you can buy keys from Microsoft, if you don't want 4 DVDs running around. So just buy one copy and then buy as many keys as you need.

You can use the same copy of windows with multiple pcs though, right? one question is what is that number? should i be looking at a system builder version or something?
im just looking for the cheapest way to do it and guarantee that it will work in all 4 scenarios and in the future

Well, legal gray area aside, using one CD key on multiple PCs is a pain. If you trigger activation too many times, Microsoft will catch on and block it from being activated for a while.