Sandy Bridge is the best choice for 10 boxing. Higher IPC, higher clock rates in general, as well as smaller die process that is more refined which can allow for higher overclocks if you do that.

I would suggest getting the 2600/2600k model and using hyperthreading. WOW, and games in general, are very monolithic single threaded applications, so the chance that a single thread might try and lock down resources are decently high. By using hyperthreading, it can allow another thread to continue to use remaining resources in a core rather than having it be completely locked down by one thread. For a single game running, this will never be a problem (and the overhead could actually reduce performance) but for five games running it can be very useful.

Oh, and WOW is definitely processor bound. It is basically a huge scripting engine that communicates via a network and draws objects to a screen. Most of the lag in the game is caused by the scripts either waiting for more data over the network, or being overwhelmed with the amount of data that is being recieved.