You can't change that later so the RPM on the drive is not a big step down compared to being locked in on the CPU. The i5 is today's place to be in my opinion.
I'm about to buy a laptop and have 2 models by the same company in mind; they are both the same price. One has an I3-350 CPU with 4GB RAM and 500 GB HDD running at 7200. The other is I5-450 with 4GB RAM and 500 GB HDD at 5400. All other features of the computers are the same; will I notice a difference in operating speed? The computer will be used for Word, Excell, email, web brousing, downloading movies, etc.