What are you using the system for?
Most people are getting 6 core CPU because of it's improved throughput of CPU intensive work like rendering. A moderate overclock, to 4.0Ghz or so, will reduce processing time further and by a meaningful amount. If the system is being used for gaming too, then the extra speed is helpful, although of more limited value. If the system is being used to generate revenue, then overclocking is practically mandatory. More throughput = more money!
'Big air' can cool that CPU, even overclocked. If you want to go with a liquid cooler, then go big. I would not use less than a 240mm radiator. You can run the fans slower and quieter.