And I would be remiss if I didn't mention that the top of the line iMac with the 3.4 GHZ option will beat the quad mac pro 5,1 in every single CPU benchmark while costing almost $500 less. Something to think about if gaming is the goal.
Now that the 7xx GPU's are out the quad can upgrade the iMac cannot. The same applies to the processor, that change will push it above the imac by the same amount the iMac beats the quad now. Mac Pro flexibility vs iMac limited modernness.