No experience with the BigWater, but I have used their Aquarius II, and it seems the only difference would be the size of the rad and fan. (Maybe?) For my A64 3200, from stock air to the AqII I think I only really saw about a 5-8C drop in
CPU temps. With the bigger rad and 120 fan, you might see an even more dramatic drop (but then again, maybe not...)
The advantage to using a kit like the Aq II/III or the BW is that with rare exceptions, all the parts you need will come in the same box... The advantage to buying kits online from, say, Danger Den, is that you can pick and choose from a variety of parts, and you can easily make sure they will all be compatible with each other. For example, the BW comes with only the
CPU block, whereas a DD kit can come with as many blocks as you may want to try to cool (
CPU, AGP, NB, etc).
How much are you looking to spend, and why are you looking at watercooling? Are you looking to overclock, and if so, how much? Very mild OCing could be easily achieved with a top-of-the-line
HSF, while more significant OCs might benefit from a wc setup.