Do you mean what should you NOT compromise on? The radiator (£60 for a PA120.3), then the CPU block (£35-£45), then the pump (£30-£75) (In my opinion). One excellent thing about water-cooling though - you can always sell the stuff on, if you buy good stuff in the first place. There is also a very healthy market in good, used, water-cooling kit. Once you get your 250 posts and access Member's Market there are lots of bargains to be had - In the last month I've bought a new, unused, Laing D5 for £35 Incl. delivery and a D-Tek FuZion for £20 Incl. and I've sold my own older stuff too.