I'm currently in the planning stages of a new rig. I know I want to run it WC'd, both for the performance and for the straight up cool (no pun intended) factor. My current rig is watercooled, but as it was a kit I'm really still a n00b as far as WCing is concerned.
So here's the question:

Exactly how important to performance is the resevoir? It seems logical that the more water in a system, the more heat it can handle, but at what point does it really start to make a difference? I'll probably be using a Thermochill HE120.3 radiator, and I'll be cooling an AMD x2 4800 and two 7800GTXs.

Because of the theme of my case, It'll require some sort of homebrew resevoir - which in turn will affect the overall layout/design of the case itself. Given the limited information that I have now, any ideas how big of a res I'll need, or if I could get away with none at all?

Any input would be great.

A res won't effect cooling after about twenty minutes or so (depending on the size of the res). Once a system hits a saturation point it achieves equilibrium, absorbing and radiating all the heat it is capable of. The larger the res the longer it takes to reach this point, but after that you can take the res out of the cooling equation.