<< I wonder this also in termsof what I believe to be called areal density. If two otherwise similar hard drives each have three platters on which to store their data and one is a 20 gig while the other is a 60 gig, seems to me that the data thats placed on the larger drive would be packed in there a bit more tightly to have the capacity of the drive increased. So if in one revolution of the platter under the drive head, wouldn't more data have the opportunity to be picked up off the platter and in turn put into use quicker than what would happen in the same revolution in the smaller drive? The smaller drive would have fewer bit and bytes flying under the read head per revolution... >>
In theory, areal density should improve performance. For instance the Barracuda IV can store 40GB per platter (higher than any other drive currently), but currently, it's not the fastest HDD on the block.
What you need to do is locate the OS on the partition that's at the beginning of the HDD (outermost partition) to get best boot times