I initially upgraded (current system) to an i5 2500K.
And overclocked that on air (Corsair A70) to 4.5 Ghz.

The system wasn't totally smooth, but was very playable even in large scale BG's.
This is with a lot of addons, playing on a pair of 27" (720x1080 resolution) monitors, Firefox with 10-15 tabs open, iTunes playing, etc..

Once I started to stream, using XSplit Player...
I noticeably had choppiness in 40-man Battlegrounds.

Upgrading to an i7 2600K (no other changes to the system)...
With Xsplit going, at stock speed the i7 was smoother than the overclocked i5 2500K was without XSplit.
Everything else what the same.
Using the exact same settings, the i7 is stable at 4.5 Ghz too.
1.34 volts, if it matters.
I used this as a guide for the OC: http://www.youtube.com/watch?v=WMZoWOpry40

For a hundred bucks extra, the i7 is a lot stronger than the i5.
The i5 likely will work fine, but the i7 will give you room to stream or run additional accounts down the road, or to play more computer challenging games down the road.
Also, every expansion... the requirements do go up; you won't be as smooth with any system in MoP as that same system is in Cata.