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Have a look at my review here for E2000/4000/5000/6000/8000 series Intel dual core comparisons. All were tested with identical components at various clock speeds so you will be able to see what you could gain.
Second hand E8400? They seem to go for around £85ish on a well known auction site.
I had one last year, ran @ 4.2GHZ 24/7 for a year or so till I sold it for £110.
You could sell your E2160 for say £30, to re-coup some of the cost.
Pre owned Q6600 expect around 3.0-3.6ghz with a decent cooler. sell the 2160 around £25 a Q6 should then cost around £80 ish
job done