Myself, I'm using a Lian-Li serverr tower case, all aluminium. You can still use watercooling regardless of what case material you use (although, you cant watercool everything, so would still recommend aluminium over other materials). The Lian-Li case also has a machine finished surface on the aluminium, not as plastic looking or having the appearance of something out of a Borg ship like some Thermaltake cases (lol). I do have a perspex side window though.
Do remember though, that while watercooling is better than air cooling (since air cooling relies on the air temperature itself to be cool enough, which is why aluminium works so well, because it conducts heat from the air inside the case), there are other alternatives.
Heat pipe technology has become very popular in computer cooling. Essentially, a heat pipe works just like a refridgerator without any moving parts. The heat pipe (which looks just like a normal solid pipe in appearance) contains a liquid which boils at the operating temperature of the CPU. Liquid in the pipe close to the CPU changes phase from a liquid to a gas, thus absorbing latent leat. If you recall your high school chemistry, in order for a liquid to change phase into a gas, it must absorb energy from the environment. This principal is used to make refidgerators cold. The gas then rises up the pipe to a radiator with a fan, where the gas cools and condenses back into a liquid. The reverse phase transition from a gas back to a liquid releases the stored latent heat, and the liquid then falls back down the pipe to the CPU again. The thermal properties of a heat pipe are 2000 times greater than a normal solid copper pipe, requires no maintenance or electricity to operate, and has no moving parts to break down. It was originally designed for spacecraft where power and space are at a premium, but the technology is finding applications in other fields.
I have a Gigabyte 120mm radiator fan on my CPU that uses 4 heat pipes to connect to the CPU. At idle, my CPU runs at 24C, and I have not seen it go over 37C under load. Mind you, I also have 6 other chassis fans moving all that hot air out of the case, which as previously mentioned is all aluminium. The larger the case, the better for cooling it is too. Hope this helps