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See here http://browser.primatelabs.com/mac-benchmarks
Your 2008 iMac gets 1654 single core and 2976 multi core (32 bit)
The 2016 15" MacBook Pro (2.5) gets 3180 and 11959 on the same tests.The new chips are way faster. The ghz numbers mean nothing when comparing between different chip generations. This link may be useful for explaining i5 vs i7 etc. http://www.trustedreviews.com/opinions/best-intel-processor-core-i3-i5-i7
My main thought would be decide if you need a laptop or not first. You get way more performance for your money if you don't need portability.
See, I use it quite a bit every day so I'd rather be using something that feels and looks nice and I am willing to pay a (certain amout of) premium for that.
I tried using cpuboss to compare the performance of the processors but pple seem to be quite vague about what the processor actually is.