i'm getting the 15" model with descrete graphics but not sure whether to spend the ~325 US dollars (British price

) on the 2.6 Ghz processor or if 2.3Ghz is enough.
they are both i7
they are both 6mb cache
does anyone know how much better the 2.6 is? is it 13% better as the 0.3 suggests or do the numbers not work like this
i do a lot of processor intensive tasks at the same time
im maxing out the ram
i dont need the $325 for anything in particular (eg no need for "spend it on x instead") but i would rather not spend it if i dont have to
thanks very very much to anyone who can help.