What is the difference between ultra3 and ultra320?

http://www.sillworks.com/specs/u320_harddrives.php (This site will give you more detail. HP SCSI Ultra320 Hard Drive Option Kits (Servers)

These Ultra320 Hard Drives are backwards compatible and can directly replace any Ultra2 or Ultra3 Proliant Universal 1" Hard Drive. The actual transfer rates of these Ultra320 Hard Drives however will be determined by the Smart Array Controller in your server (U2/U3/U320).

A wide variety of rigorously tested, HP-qualified, SMART capable, Ultra320 Hard Drives offering data integrity and availability in hot pluggable and non-pluggable models. HP 15,000 rpm Hard Drives provide up to 26 percent better performance over standard 10,000 rpm drives. Actual performance increases will vary depending upon application and configuration.

All HP hard drives pass a rigorous qualification process, which ensures that every HP drive is proven to perform in your ProLiant server or workstation. Before final certification, all HP hard drives are provided with HP-unique firmware specifically designed to optimize compatibility and functionality, including support of SMART and Insight Manager.

HP received the 1999 Gold Idea Award for design Excellence for the innovative design of the Hot Pluggable Universal drive carrier. The carrier is praised for both its ergonomics and aesthetics, as well as its many functional enhancements such as the compact design, improved cooling properties and more secure drive mounting for reduced vibration.