If you are purchasing 5 computers you most definitely do not need a 64-bit OS.

Currently WoW doesn't need much "computer power" to run. You can get pretty much any new computer and it will run fine on it. I'd say anything with at least 512MB RAM, 80G HD and a dedicated video card (not even needed really) would work fine. I'd aim for the $300-$350 mark for a computer. I am sure you could even get some for $150 refurbished and they would be fine for slaves. Check newegg.com for some systems. I don't live in the USA so I don't really know where else people shop for good values on computers. Tigerdirect.com maybe?

Of course, I would likely go with 1 or 2 decent computers instead of the cheaper boxes. It really depends on what you like and if you plan on focusing on PVP or not.