What do you want to cool (just the CPU? If you're using the relatively cool running when-overclocked Sandybridge 2500k or 2600K, you might want to consider saving money and going for an easy to fit Noctua NHD14 or a Corsair H100; the advantage of decent water cooling loop would be a little less noise under load; even when you change the H100 fans for something better in push-pull config, under load you can apparently still hear its pump) what sized radiator can your case take, etc. If going for a water cooling loop to cool your CPU to a level that might prolong its life a little a bit, I would go for something like the XSPC Rasa 750 RX360 WaterCooling Kit at around £200. The cheaper RS version at around £140 uses a thinner radiator equals higher fan revolutions equals more noise under load. The RX version is a good upgrade point at a later date because the radiator is a keeper. Again, you have to determine whether your case can fit the 360 radiator, though the RX240 at around £170 could be a good cost/performance/size compromise if you just want to cool your CPU alone. If you want to use water cooling to cool your graphics card and possibly even other parts, the price rises very sharply and there are a lot of overclockers forums you could check out.
Edit - a quick look around and it looks as though the XSPC Raystorm EX kit series has superseded the Rasa RX kit series. For instance, the XSPC Raystorm 750 EX360 WaterCooling Kit is around £200.
If it's me, I want to get a quiet case and parts that review as being quiet, from the get go, so that the need for water cooling isn't necessarily there. A stock H100 and a sensible 24/7 overclock level on Sandybridge might be enough for you - you can check out the relative noise via youtube videos - and you can make it quieter at a later date, though unfortunately not necessarily better performing, by changing out the fans for quieter models at the same rpm.