How much improvement from C2D to i5?

my friend offering me macbook pro cheap price because switching pc. own 2.66 c2d macbook pro 4gb ram. macbook pro brand new last week 2.4 i5 4gb ram. how improvement c2d i5? make sense me upgrade? i'm familiar graphic cards , such.

thank you!

 

if computational needs benefit speedup, maybe have here: mac benchmarks geekbench

 

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