A standard issue 5box machine is basically:

Q6600 Processor
4gb RAM
Good Motherboard
Geforce 8800GT / 8800GTS / 8800GTX
600W+ PSU (good brand)
1 or more Hard drives.

going with a 8800GT and 1 hard drive, I made my machine for $850. You could make it beefier with a 8800GTS/GTX and more hard drives and still be well under $2k.

Edit: The main "disadvantage" to 1 machine vs multiple is the possibility that at some point Blizzard will ban the software that allows us to multibox. They don't seem to have a problem with it, but all the hardware-boxers have a) this conspiracy theory b) enough money to buy 5 machines anyways or c) both.