For overclocking motherboards, you want more VRM phases. The short explanation: typically, more power phases = a more stable, higher, and cooler overclock. You can read more about VRMs here: http://www.hardwaresecrets.com/article/Everything-You-Need-to-Know-About-The-Motherboard-Voltage-Regulator-Circuit/616/4
Now, this doesn't mean you need to buy a 16 phase motherboard. Good motherboards have 8 + 2 power phase design. When I'm looking to find out the power phase for a motherboard, I simply search "<make & model> power phases". There are also forums where people have complied lists with motherboards and their power phases. (Overclock.net http://www.overclock.net/t/946407/amd-motherboards-vrm-info-database).
The reason I'm telling you this is because you can find 8 + 2 phases on motherboards from $140 to $250. Both can achieve similar results.
Overclocking takes some time and luck. Each CPU is different. Just because you've read that Person-A achieved 4.8 GHz, doesn't mean you will. OC'ing is a trial by error process. There are many OC'ing guides that you should read to understand the principals so you know how not to damage components in your computer.
I understand you want to water cool your system. But, do you have a budget for parts?