If you're planning to install the Mac OS X beta for day-to-day use, expect a dose of disorientation. You're installing the beta because you can't wait to live in the Aquafied world of Mac OS X, right? It ain't gonna happen. In fact, you'll probably spend most of your time in the Classic environment, which looks and behaves pretty much like Mac OS 9--though everything runs slower in Classic than it does in true OS 9. (See the results of Macworld Lab's tests at http://www.macworld.com/2000/09/13/lab.html.) And a lot of things don't work.
At best, you'll get bounced back and forth a lot between Mac OS X proper and Classic, because few popular apps will be Carbonized--rewritten to take advantage of OS X--until well into the beta period. See http://www.versiontracker.com for a constantly updated list of Carbonized apps.
In the meantime, here are a few pointers to help you survive the ride.
Networking No matter how you configure your network settings in Mac OS 9, Mac OS X will override them with its own settings when Classic starts up. Mac OS 9's networking software, Open Transport, is disabled by OS X. Location Manager may appear to be active in the Classic environment, but it doesn't work. Likewise, you can't use OS 9's Remote Access control panel to make a dial-up connection to the Internet. You have to use OS X's PPP Connect application. And AirPort isn't supported in OS X beta, period.
Fonts Mac OS X and the Classic environment don't share a common set of fonts. Your Classic apps will have access to all the fonts you had installed in Mac OS 9, but your Mac OS X apps won't, unless you also install them in the OS X Fonts folder (System: Library: Fonts).
Extensions and Control Panels They're not accessible in Mac OS X, and they don't all work when you run Classic apps. Before starting up Classic for the first time, you'll probably want to turn off all third-party extensions and control panels, leaving active only those that were installed on your computer as part of Mac OS 9. Once you get Classic working properly this way, you can start adding back your favorite extensions, checking each one to see if it behaves itself in the Classic world.
Peripherals Printers, scanners, mice, and other peripheral devices communicate with your computer through bits of software known as drivers. Well, guess what? Mac OS 9 drivers don't work under Mac OS X, so peripherals need all-new drivers for OS X. Many of these aren't ready. Apple provides some generic drivers for devices such as laser printers, mice, and keyboards, but you'll probably have to wait awhile to use all of the features of all of your peripherals with OS X. Most notably, the Mac OS X beta won't let you print to USB printers from Classic applications.
Don't be disheartened. The OS 9-to-OS X transition is going to be a bit rocky. As long as you're prepared for a bit of bizarre behavior, you'll do fine.