Anyone building a new rig at the moment would be totally insane to buy anything other than a Core i7 2600K .
It blows away all the old school i7 CPU's, is multiplier unlocked (overclocking to about 4.8 Ghz aircooled), and is as cheap as, especially considering its pretty much the fastest kid on the block even without overclocking all at a mid range price.
http://www.guru3d.com/article/...re-i7-2600k-review/1
http://www.anandtech.com/show/...-core-i3-2100-tested