It all depends on how much money you are willing to spend. The bare minimum for a pretty decent machine is about $500-$600. But, you could even go up to $2000 for a beast. Budget is a big factor.
This is as good as you will ever need. http://pcpartpicker.com/p/2EA3z
If you want to go up to 2100 dollars upgrade from the i5-4760K to the i7-4770k and upgrade from the 780 to 780 ti.
DON'T FORGET YOUR MONITOR! LOL