A few other considerations:

  • Do you like using PiP? Can't do PiP across multiple computers.
  • Movie recording. Your PC might be able to 5-box but can it 5-box AND record movies? If that's something you'd like to do, I'd recommend two PCs.
  • Redundancy. If your 2nd box can run 4 WoWs, it can probably run 5 with lowered settings. Meaning, if you lose one of the boxes, you can still multibox while waiting for your NewEgg shipment to arrive.
  • KM switching. As others have noted, you'll need a KM software app like Synergy (not recommended due to noted bugs), Input Director, Multiplicity (not recommended due to the way it handles output when on your 2nd PC) or even Octopus so you can mouse over to an alt on the 2nd PC and type/drive/whatever.
  • Size of Alts. If you use 2 PCs with a 22" or larger monitor, your alts' windows will be big enough to actually see chat and such, depending on how you have 'em configured.