Are you on intel platform? Normally a 8400 will pair up nicely with the thx 1066.
If you want to buy a k version, 8600k/9600k is a good choice. Most 9600k can hit 4.8 with reasonable voltage. However in order to hit higher clocks, you would need 9700k or 9900k or 8700/8086k. Reason being i7 has a better asic quality than your i5s. And is able to hit higher clocks given that you are able to cool it. Also you would need a stronger mobo for overclocking the i7, as the power draw is higher for i7.