You can rig out a basic machine to run at least 3 accounts. That's my theory, anyway. Nvidia's SLI x3, pci express, dual core processors, motherboards capable of 16 gigs of ram (windows only supports 4 gigs though so don't go overboard). But it all starts with the motherboard, just be sure you get one that you can add upgrades to. And begin with a decent power supply.

For the most part it's cheaper than ever to build your own machine. As for me, I bought one piece at a time as I could afford it, but I made sure to get the best piece I could find at the most reasonable price. I'm going to sound like a walking advertisement, but I found the best deals on newegg.com and numerous rebates, saving me over 100 bucks and spending around $600.

My two accounts play smoothly on it, and I'm sure I could slap in an additional video card and a couple extra gigs of ram for about $150, and easily play another two accounts. If I needed more processing power I could find a speedier AMD chip. At present I got the AMD Athlon 64x2 dual core 5200+ (but after that speed the prices started rising exponentially).

I've got a single nvidia geforce 8600 GT 256;
2 gigs ram, AMD Athlon 64x2 dual core 5200+,
Gigabyte MoBo with 2 Pci Express slots and upgrades to 16 GB ram
(Would have held out for 3 PCI-E but having 3 cards was just unfathomable to me then).

I already had the monitor and dvd/rw drives, and peripherals.
Two WoWs at max settings run beautifully, and I expect a third even with my current configuration would be alright if I lowered the settings a bit in WoW. With another vid card and more ram, no xtra graphics settings, I could see 5 toons on the same box. Could be wishful thinking and probably is.. But hey I can dream =^)