Go for cooling your
cpu, then gpu's unless you have a hot running north bridge i wouldnt bother water cooling it, same for the
ram. You will need to keep some airflow in the case as things like voltage regulation circuits will overheat, and the
psu will suffer as well. The more you add to the loop the bigger the pump and radiator you'll need.
If your room is on the warm side go for at least a double 120mm rad if not two of them. or even a tripple 120mm rad.
Eg, in the following loop
Res -> pump ->
cpu -> rad1 -> gpu's -> rad2 -> back to res
This was both the
cpu and gpu's will have the cool incoming water. (Although
cpu will run slightly cooler in the following arangement,
res -> pump ->
cpu -> gpu's -> rad1 -> rad2 -> res,
the gpu's will run hotter. This is because the input water to the first rad will be hotter and thus more heat will flow due to the higher temperature difference)
its worth nothing that 2x double 120mm rads are cheap than 1x quad 120mm rad.
I use the thermaltake bigwater series kit, its cheap, cheerfull and has done me well and its currently keeping my quad core
cpu, and x1900 nice and cool