If you want to avoid the cost of a new motherboard/DDR4 ram/CPU then yes, upgrading your current CPU is the most cost effective way. You could drop an i5 4xxx or i7 4xxx in there, depending on the motherboard CPU support you have. The 'K' processors are meant for overclocking. You are an overclocker, but I'd remind you that your main benefit in upgrading is gaining the extra cores and threads. So overclocking isn't really a priority.
Myself, I'd favor an i7 if possible, because games are already making use of more than 4 cores/4 threads. However, an i5 is still going to get the job done assuming you have a decent videocard. This all depends on the software you run of course. The more cores the game or software uses, the more benefit you'll get from a CPU upgrade. If you play less demanding games then you might not see any benefit. If you are videocard limited, you might not see any benefit.