is overclocking is harmfull ??

The only way oc would reduce components life spand is if u'd run them at their max. temps. If u have like a corsair H100i cooler on the cpu and u oc it,u wont do any harm to it. But my advice would be( thats if u play mostly multiplayer games) go for more then 4 cores cpu, if i7 is over ur budget then rather get amd something like 8370 or 8350. Cuz an i5 in a game like bf4 with 64 players ur cpu is gonna run to the max! i know cuz i had i5 3570k and now i own 3770k. To anwser ur question, yes the K version of intel's cpu is worth buying! But also make sure u get an aftermarket cpu cooler. Oc'ing the cpu on a singleplayer game wont change anything, but in multiplayer games, yes oc it and oc it good! it will make a big difference. hope my comment was helpful. if u have any questions , feel free to ask

If you run it at max speed and without adequate cooling then you could risk frying it. Otherwise make sure, if you're going to overclock, that your using either a very good heatsink/fan or water cooling. Noctua makes a few but they are huge and require a big case. If you want something more compact but will overclock well, look at going with a water cooling setup. With water cooling you don't need a huge case, but either way you do need to research out the best option for your specific case. Keep in mind the Haswell lineup of CPUs dont overclock quite as well as previous generations.

As for the K vs non K, unless the K version is priced severally over the non-k version it'll be worth it. There's are couple of reasons.
1. Even without overclocking the K version runs at a higher base speed. 4Gh for the K version, 3.6Ghz for the non K. This also translates in to a 400mhz jump in in turbo.
2. The K is the new Devils Canyon refresh that Intel put out for the new Haswell refresh. They revamped the lid and it provides better cooling than the non K 4790.

Something to consider however; do you need an i7 processor? Most gaming (tho not all) are single threaded applications. Depending on the game an i5 may out perform an i7. Look in to the games you want to play and see if they're single threaded (i5) or can take advantage of an multi threaded cpu (i7). While most people think the biggest most expensive CPU will do the best in games, this isnt the case. Its comes down entirely to what the game is designed for. Your video card, RAM and network speeds will also play a significant role as well, so its important to know what the components of your build can do. Now if your planning on doing more than just playing games (photo editing, video editing, Virtual Machines) then looking at an i7 is a good idea, even if you might take a small hit in some games. If you find the games you want to play work better with an i5, still go with the Devils canyon refresh if you can so you get the benefit of the better cooling and speed increase that came with the refresh.

Even if you decide you don't want to overclock, I would recommend going with an aftermarket cooler. Something like the Cooler Master Hyper 212 EVO can be had for about $35. I have the 4790K and I tested it out with the stock cooler and an aftermarket cooler. At maxed out speeds, admittedly something you don't see outside of stress tests, the stock cooler would throttle CPU speeds instantly. Intels stock coolers while they work aren't very good and should be swapped out if your concerned about performance and heat issues.

No, a proper overclocking and cooling doesn't make the components die. The heat due to overclocking and unnecessary voltages are the main causes for damage of components.