My advice has always been that if you (or whomever else) are going to use a chart (or benchmark) that was built to help people understand how a system can handle a single game client, that you should lean toward the higher end of the chart in order to gauge gameplay performance while multiboxing. That, and the fact that if you're going to build a multi-GPU system that you should stick to nVidia at this time since Crossfire doesn't work in any windowed mode.

That site also has a breakdown for World of Warcraft specifically, but to be honest I haven't read through anything they say on that page, so I cannot comment on that at the moment.

If you want more advice on what you should get, then we'll probably need a budget to start with.