Because you have a E6600 @ 3.3ghz might not be worth jumping to another dual but going quad instead, E6600 @ 3.3ghz is still fast.
But for £160 best you can really get is E8400 and you could hit 4Ghz with it.
Best quad with good OC'ing would be any of these but will cost you bit more:
The 9550 is a very popular chip, allot of guys been hitting 4ghz + with those.
What cooler you using?
Review of Q9400:
For money you want to spend 9400 would be good pick, but if you can push to the 9550...
Ouch the quad cores are not cheap but the first one seems good
is that a gud one?