You have a Z87 motherboard and a "K" edition CPU, which are both designed for overclocking. Heck, you even have a Cooler Master aftermarket cooling solution, so you should definitely be able to overclock your CPU to at least 4GHz. Whoever curated that list clearly had overclocking in mind. You should overclock; whereby you will gain a performance boost anyhow without a new CPU. Go into your BIOS, go to your CPU section, and look for something which states "CPU Clock Ratio" or "CPU Multiplier." If it's set on auto, set your clock ratio to 34. From there just bump it up to 35, and restart your machine. Then keep bumping it up until your system is not stable. However that's an old method.
I'd advise you just set your clock ratio to 40, and run Prime95 for about 30 minutes to check if your system is stable. If your system blue screens or freezes (which indicates the overclock is not stable), you may need to reduce the clock ratio down to lets say 38. However at 40 you should still be stable. Since you'll be stable at 40, try incrementing up the clock ratio to 41, 42 etc. and keep experimenting on which is stable.
If you set your clock ratio to 40, with the i5-4670K your CPU will be overclocked to 4.0GHz, and at 41 you'll be at 4.1GHz and so on. Most people using the same hardware as you, even the same cooler can stably overclock to 4.4GHz (setting the CPU ratio/multiplier at 44) which are at stock voltages. You don't need to increment stock voltages if you're not comfortable with overclocking; simply set your CPU ratio/multiplier to 44, or whichever is stable for you.
This thread may be of assistance, or you can look up overclocking guides online.
All the best.
AMDRadeonHD: What's wrong with the PSU? 550W is enough for a R9 290 if it's not overclocked. The XFX 550 Pro is a solid PSU.