There is no way you will notice a difference of .13 GHz. In benchmarks you may see a slight advantage to the faster CPU but in practical terms you would not notice anything unless you were running some kind of crazy CPU bound calculation that took hours of full CPU usage.
You could, however, notice a difference between the different GPU's offered when a 3-D gaming.
I just ordered and awaiting my i9300. While configuring I selected the 1.60GHz CPU. Most people will NOT see any difference between the 1.6 and the 1.7 and its simply not worth dishing out the extra 200.00 bucks to go with the 1.8. IMHO you'd be best off purchasing the lower end CPU and waiting 6 months at which time buy the 2GHz CPU for 150.00 and sell the replaced unit. Exactly what I plan to do!