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When it comes to computers I seem to suffer from the same sort of need to upgrade (even I don't actually need to) that I experience with bikes. I have a reasonably good PC that's a few years old and running an Intel i5 and it doesn't seem to struggle with anything I throw at it. The motherboard is an older socket 1156 design but you can still get 1st generation i7 processesors.
Would an i7 offer much of an improvement over a similarly clocked i5? I appreciate that the i7 has hyperthreading technology (giving 8 threads over 4 for the i7) but at the end of the day you've still got your 4 core whether it's an i5 or and i7; I'm not interested in the i7's onboard graphics.
I don't think it's worth it. If you are going to upgrade you might as well change motherboard to 1155 and get a major performance boost with the 2nd and 3rd gen stuff. Saying that a first gen i5 is still more than capable and better than 75% of the CPU's you can buy new right now
What software are you running?
My understanding is that i7 is only remotely worth it, if your applications use the hyperthreading. Games don't really, and web browsing/email ect doesnt. A spreadsheet/wordprocessor is unlikely to get it anywhere close to capacity. If you a lot of graphically demanding stuff like CAD or Video encoding, then apparently thats where i7 begins to pull ahead, but unless you do mostly that, I'd be inclined to save the cash.
I have a Dell Precision 4600, that work gave me. I gather its got an i7 processor. It certainly doesn't struggle at all with 3D CAD & FEA work.