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I've just changed from a q9550 @3.6ghz to a 2600k, there is a noticeable change. Even though my Q9950 was rapid! My new setup wipes floor with it. The games load so fast, a don't know what you use your system for but for example , I encode divx to DVD. On my Q9550 I could encode a film at 200fps so took around 5 mins. I'm now encoding at 600fps and takes about a minute! Because in windows the hyperthreading acts like its an 8 core CPU! I'm very pleased with upgrade 

. Though I suspect we will have 24 core cpu's as the norm by then