The nice thing about getting a K-series processor is the unlocked multiplier. Your processor speed is your BCLK (base clock) multiplied by your multiplier. Since all Sandy Bridge CPUs are stuck with a BCLK of 100 (you could probably tease it up to 103 if you really wanted to but it I would just leave it at 100) the only way to overclock is to up the multiplier. You could easily over clock to 4.0 Ghz (multiplier = 40) and most chips will do 4.5 ghz no problem. I would probably start around 4.2Ghz (multiplier = 42) and see if your system is stable. If it is, you can either up the multiplier and see if its still stable or, if you are happy with the performance, just leave it at that. Just watch your temps.
There are a lot of guides on overclocking out there. Look some info up, but beware because overclocking gets kind of addictive. You just keep wanting more and more speed lol. If you aren't careful you'll end up spending all your money on liquid nitrogen cooling and exotic motherboards.