The Serial ATA150 Raptor 10K RPM drives would be your top choice, though they don't give you a very good price per storage ratio. Because of this, I wouldn't recommend them at all. I'd just go with a nice SATA3.0 7.2K RPM drive.
Depends on how many users for how much storage you'll need. I personally wouldn't think 320 GB would be enough... but it just depends on how much you think you'll be storing.