The Advent mentioned seems the best option.
If you get it from PCW don't forget to use Quidco for 2% cashback (internet and 'collect at store' purchases only).
PC World (and Currys for that matter) have a price match promise: http://www.pcworld.co.uk/martprd/editorial/price-match-promise
so buy via quidco, collect at store, then see if you can get it cheaper at other retailers on the list - esp if buying down at Brom' as there is Comet, Asda and Tesco Home on the doorstep.
FWIW if going over 3GB RAM you start to need a 64bit OS. In fact MS have limited the 32bit versions of Vista to only see up to 3.12GB RAM by default.