It does make a huge difference if you're comparing smaller drive sizes. For example, a 7200 RPM 320GB WILL outperform a 5400 RPM 320GB drive.
It gets a little trickier when you look at the larger ones. I'm too lazy to find you the links but there were some studies done where they compared a 500 GB 5400 RPm to a 320GB 7200 RPM drive. Performance was similar, and in some cases, better.
It's got something to do with the larger platters on the bigger drives being compressed into a smaller space. Someone else can explain it.
What it comes down to is that yes, drive speed makes a difference but not at the larger sizes.
That is why I recommended a 500GB Scorpio BLUE. It's about 10 bucks more than a 320GB Scorpio Black. Same performance (because its larger) with less noise, vibration and more battery life.
eV
here you go ... same discussion from last night
https://forums.macrumors.com/threads/734273/
Someone needs to make a sticky on this topic!