I'm playing on an i7 920, at stock speed.
Have 12GB of DDR3, 1600mhz.
A GTX275 1MB video card.
And a Patriot 64GB SSD (180mb/sec).

Dynamically, my main uses medium settings mostly.
Shadows and Weather disabled, View distance maxed.

My slaves use minimal everything, except medium view distance.

Nothing is overclocked, and the game is playable everywhere.
Stormwind is smooth with 80 players around the AH on a weekend afternoon.
As are all battlegrounds.
Mass PvP kind of pushes the system a little.

You could get a much better system, for comparable price.
Mine is more than a few years old now.

The i7 2500k or i7 2600k processor.
Some ram.
A decent SSD drive for the gaming folder.
A video card, that was new within the last 2 years.
Should play the game quite smoothly anywhere, as a 5-boxer.