I'm doing it almost exclusively for noise. I have a 6990 and its incredibly loud. its taking a while (and took a back seat for a bit), but I'm doing it on the cheap almost exclusively second hand.
so far I have:
RS240 rad (£20, unused)
swiftech epsilon 6990 block (£22, like new with unused thermal pads)
Laing D5T strong, (£55)
EK pump top (Unused, Free)
8xbitspower compression fittings, (£4, 6 of them unused)
assorted O-rings, £3
4xGT 1850rpm 120mm, (£35 new)
all I need now is an xspc raystorm CPU block, a Res, tubing, another 240 rad, some more compression fittings and fan controller, then I'm good to go. I'll probably get them all together, but my motherboard is away on RMA cause a RAM slot died.
point is, if you're willing to be patient, water cooling doesn't have to be that expensive. Most of the parts will last me a long time (hopefully), aside from the GPU block, which will have to be upgraded with every GPU obviously, everything else should stay the same no matter what I change out.