I'd go with 6Gb.
This is a good buy:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-048-KS&groupid=701&catid=8&subcat=1389
There's a Patriot Viper kit that's even better value IMO, unfortunately OcUK don't sell it.
This is a bit cheaper and has better latencies.
http://www.overclockers.co.uk/showproduct.php?prodid=MY-129-OC&groupid=701&catid=8&subcat=1389