As mentioned by logainofhades, your motherboard does not support overclocking, where the "K" edition CPU's are designed for overclocking, hence have an unlocked CPU multiplier. The "K" suffix on the i5 you desire to purchase denotes it's for overclocking: i5-3570K. The i5-3570 is not for overclocking, notice it does not have a "K" on the end.
Your motherboard does not support overclocking the CPU either, so it would be pointless to purchase a K edition CPU whilst paying more for it too. Your motherboard has the H61 chipset, which is not overclockable. If you had a P67 or Z68, then you could overclock (motherboard's with the prefix "Z" are designed for overclocking, i.e. the Z97.)
Since your motherboard's chipset is H61, it was released with Sandy Bridge (2nd generation CPU) support. The i5-3570K is Ivy Bridge (3rd generation), which is still supported with your motherboard only if you update your BIOS. You can tell the i5-3570K is Ivy Bridge by looking at the first number after the hyphen; i5-
3570K, the 3 denotes third generation which is Ivy Bridge.
As mentioned Ivy Bridge is supported with your motherboard, only after you update to the latest BIOS released by your manufacturer, in this case MSI. You can find more details about your motherboard
over at MSI here, and you can also
downloaded the latest BIOS from MSI too here.
Also as mentioned by logainofhades, if you're willing to spend that much money on an i5, you'd be better off purchasing the Intel Xeon E3-1230 v2; which provides similar performance of an Intel i7-3770, but at a lower cost. Again to use the Xeon you'll need to update your BIOS. It's worth noting that the Xeon does not have an integrated GPU, so you'll need a dedicated GPU to use it; your R9 270x will work great. With that said, you won't be able to boot up your system with the Xeon if you do not have a GPU installed; regardless the Xeon is still worth the price per performance.
All the best.