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Any consideration being given to the Q9550? or is the i5 simply in a class of its own?
Without changing your mobo, best bang for buck would be an e84/5/600 from the members market. 4Ghz for ~£80.
I'd say keep the quad, but I'm making the change myself from a Q6600 to E8500 today - then again, my quad only does 3.3
I have an E8400 @ 4ghz and a q6600 which i run @ 3.2ghz day to day. I get better results from the q6600 and generally is an all round faster cpu. Running the quad at 8x400 gives better results than the e8400 in all benchmarks.
I would keep the Q6600..
My main machine is primarily for gaming - I'll see much better results with a 4ghz dual than a 3.3 quad in most games (especially the games I play - UE2/Source/Q3 engine stuff).