Been there, done this.
It worked great, the 4c water temps helped drop my old XP2500 bartons temps by a huge amount, something like 17c iirc.
As for the pressure, well if you manage to "blow the top off of your block" then you would have to be a totaly disfunctional motherhubbard, you use the tap itself as a pressure regulator, when you see the hoses start to swell with the pressure, you know you're near the limit. Plus remember taps in every house have a different pressure, my new house has pathetic tap pressure, where as my old house was like an h2o eruption.
As for being green, well I run my system like that for about an hour and probably used no more water than the guy next door to you uses when he's watering his plants, sprinkling his lawn or washing his car/wife.
Since when did overclocking and tree hugging go hand in hand? just think of all the heat polution and extra electricity costs/production an overclocked PC uses compared to a stock one.