When you say "only" two platters, it sounds to me as if you're implying that with fewer platters there is lower performance, which is not the case. In fact, while transfer rates remain the same, drives with fewer platters as a rule have slightly faster seek times than their bigger siblings from the same series - though it's so minor that it does not translate to any real-world benefit.

The 640GB is *significantly* faster than the 750GB. For starters, it's transfer speeds are roughly 120(max)/93(average)/60(min) versus 95(max)/77(average)/50(min) on the 750GB. Seek time is pretty much the same. I googled and could not find a direct 640GB vs 750GB review unfortunately, but which of them is faster seems clear.