What I got from reading the linked threads, from my own experience and from other posters on this thread was that, as long as your graphics card is not bottlenecking your system, overclocking should make a difference for gaming. Whether the difference is noticeable will depend on how much you are able to overclock and whether the particular game you are playing is able to make use of the extra CPU speed. Another consideration was the comments from people who believed that the additional performance gained by overclocking their CPU meant that they were able to postpone replacing their entire system. Of the negative posters only a few seemed to have any actual experience and this seemed to be for specific games that were known not to be coded to make full use of the CPU.
I'm not sure how realistic the extra £200 figure is. An OEM 4670K is an extra £12 over the retail non K, a decent mid range cooler (TS-140) is £35, and the difference between an H87 and a Z87 board can be around £30 depending on what specs you need but you might have been planning on a decent board anyhow in which case there may be no price difference. I'd suggest the most you'd *need* to spend would be an extra £100, which would potentially get you a quieter more efficient cooler, a better quality motherboard, increased performance in some games and a system that may not need replacing as quickly.
If you're still not sure I'd make a list of the games you play most often and then go to their specific forums to ask whether an overclocked system would make a noticeable difference.