What about just putting all the fans on the middle setting? Surely that would help without being too noisy. As far as fans go probably the quietest ones that still move a lot of air are the Scythe Gentle Typhoons.
Forget about liquid cooling for now because of the cost. With a double GPU setup the cost of the blocks alone would be in the region of £160 and then there is the pump, reservoir and radiator plus odds and ends.