This is a guide to overclocking that processor http://www.tomshardware.co.uk/forum/265056-29-2600k-2500k-overclocking-guide
Overclocking means to increase the speed of your processor by overriding some of the settings, processors with a "K" on the end are designed for overclocking. Overclocking can significantly improve performance but you usually have to supply the processor with more voltage and this increases its running temperature.
If you don't plan to overclock get the 8350. With the 8350 paired with a 970 motherboard you will get some extra stuff like SATA III and USB 3.0 support unlike the older Intel motherboards for that Sandybridge processor.
Sandybridge is just the name of the processor series, the latest Intel i5's are called Haswell and the new ones due out next year will be called Broadwell.
Finally, both those will play CS:GO nicely