if you're buying an AMD system, you'd be INSANE not to go with an nforce 2 motherboard. if you want features like onboard sound, lan, serial raid, etc, go with the Abit or Asus versions. if you want pure raw ocability with very few features for a cheap price, the gigabyte board is for u.
as for watercooling that system, you should have plenty of headroom in price. therefore, i'd go with a White Water block from Cathar, a DD GF4 cooler for video, might as well get a chipset cooler from DD since you'll have the cash to burn. toss all that on an eheim 1250 with 1/2" tubing/fittings for everything and a Dtek heatercore with shroud. grab yourself a 1/2"T fitting from DD or Dtek, a 1/2" barb from home deopt, along with a faucet/toilet stopcock valve with a 1/2" fitting for the end of the T to allow filling/bleeding, and you're set.