If you're a Pro Tools LE user, you won't be able to upgrade to the newest flavor of Pro Tools (version 9 HD). Pro Tools 9 requires a Pro Tools HD iLok software license for the upgrade package. As an LE user, you should upgrade to Pro Tools LE 8. This upgrade is available at Sweetwater.com (see Resources).
If you're an HD user, you have several upgrade options. You can upgrade to Pro Tools HD 8 at Sweetwater.com (see Resources). You also have the option to upgrade to HD 9. The cost of this upgrade varies depending on which package and included options you choose (downloadable, boxed and add-ons). If you're planning on upgrading to Pro Tools HD 9, you must have an AVID-qualified Mac or PC. Even if version 7.4 HD runs on your computer now, that's no guarantee that version 9 will. You will need to meet the hardware requirements to run it; for PC users, you'll need Windows 7 (see Resources).
Pro Tools is a music and audio production hardware/software platform for PC and Mac. It combines Digidesign/Avid hardware designed for Pro Tools software, and the two work together in tandem. Pro Tools 7.4 was the standard version used from 2004 to 2007. In late 2008, version 8 was released in two flavors, LE and HD. In early 2010, Pro Tools 9 HD was released, which is no longer offered in an LE package. The difference between LE and HD primarily is the number of tracks it allows you to use in your mixes within the program. HD also supports more recording inputs and a wider variety of hardware approved by Digidesign/Avid.
Pro Tools 7.4 has been the industry standard for a while, but it's an aging version of the software. Version 8 has some new features such as the all-new interface, track compositing, new MIDI editor, score editor, mini grand plug-in, Xpand2 and Boom drum machine. If you choose to go to version 9 HD, you'll get all the 8.0 features and some new plug-ins, plus greater number of tracks and simultaneous record voices, busses, instrument and aux tracks. Version 9 also has Automatic Delay Compensation and Core Audio/ASIO support; version 8 doesn't.