YES! A LOT! The i5 3570k at stock is 3.4ghz but will boost up to 3.8 without volt change also the i5 3570k has L3 caches which are VERY fast. The q9400 doesn't have L3 caches. I RECOMMENDED getting the i5 3570k!
thank you for such quick reply
i read a lot about a big difference between my old cpu and i5 3570k but some people confused me with their advice to not upgrade (because, as they say, there is only 20% difference)