I just bought a NZXT KRAKEN X61 AIO Water Cooling Unit and am very happy with it.
http://www.overclockers.co.uk/showproduct.php?prodid=HS-014-NX&groupid=701&catid=2330&subcat=2347
- build quality seems very good (except plastic backplate)
- it is surprisingly quiet - I have it running on silent mode and can't hear either fans or the pump (only heard the pump the first time I installed it)
- temps are 26C at idle and haven't been above 40 at load (cooling a 4790k @ stock) - maybe summer will see higher temps TBC, but then I can change from silent mode if need arises
- software is easy to use/install
- these kits are a bit expensive (although OCUK didn't have it in stock I got it for £100 elsewhere) they are worth it since they are not massive like air coolers which are dust magnets and make it hard to work in case ie access RAM sticks etc... as well as lot less strain on motherboards
as for the case it's personal choice - choose one that fits your budget, styling, size and functionality criteria. my main tip for silent case is to make sure you get steel over aluminium. there is less resonance. furthermore you can add sound dampening kits for the case that you like best
http://www.overclockers.co.uk/productlist.php?groupid=2362&catid=1170
if you want absolute silence then you need a fanless setup, although not sure how well those do for gaming etc... but are conceivable for an HTPC
my 2 cents
PS here is a good source if silence is important for you
http://www.silentpcreview.com/