if you 100% are not going to overclock i wouldn't pay over the odds for either an e8400 or q6600 as i think your probably gonna pay at least £100 for the e8400 or god knows how much more for the q6600 and be left with either a 3ghz dual or a 2.4 ghz quad 
in your situation i would have to go for a phenom 2 x4 955 and a 760g motherboard which you will get for around £160 of ocuk.
that will mean you have a 3.2 ghz quad and be able to use your ddr3 that you have and you could sell your current motherboard to offset a bit of the cost.
i would never normally suggest this but as you state catagorically that you will not buy second hand and will not overclock it is the only purchase that would make sense to me.

in your situation i would have to go for a phenom 2 x4 955 and a 760g motherboard which you will get for around £160 of ocuk.
that will mean you have a 3.2 ghz quad and be able to use your ddr3 that you have and you could sell your current motherboard to offset a bit of the cost.
i would never normally suggest this but as you state catagorically that you will not buy second hand and will not overclock it is the only purchase that would make sense to me.

