|If your motherboard SATA controllers only support SATA I you need to verify any drive you buy is backward compatible. All are not.|
IMO Hitachi drives are not in the same class as Seagate & Western digital.
As far as drive size goes you may not be aware of why hard drives are as large as they are. Below is a link to newegg.com. I have filtered down to certain brands and sizes. Use the power search and you can filter to your hearts content.
Verify about the SATA II stuff if that is a concern.
It is all about arial density. That is the measure of how much data can be written to a given space.
This impacts the speed of a drive because higher densities mean more data can travel under a head in a given time frame.
So, in many cases the larger drives are also faster. Then the larger drives are coming with a bigger cache. Some up to 32MB.
I have been using WDigital lately and have had good luck with them.
One other item to note. NCQ and the Western Digital equivalent speed up the drive by putting the seeks in order so the head doesn't move as much. That cost a slight premium but it does work if your board can support it.