Have a read here:http://www.tomshardware.com/forum/372998-31-3570k-7870-8320-7950
The 3570k beats the FX4300 hands down and more or less the same as the FX8320. But it all comes down to your budget.
My recommendation (depending on your budget) will be:
-3570k (+ after market cooler if you are thinking of overclocking)
That config should last you a few years and you can always add a 2nd 7870 a year down the line.