What's different about Mac OS X?
Mac OS X offers improved stability and reliability (but frankly, OS 9.2 is pretty stable, too). Behind the scenes, it is a more "modern" operating system with some of the new features people have asked for, like protected memory and preemptive multitasking. Among other things, this means that if one program should crash, you won't have to restart the computer. Also, a program will not hog your entire system (while starting up or performing a complex calculation, for example). Mac OS X also has numerous changes to the user interface, making it easier to do certain tasks.
Mac OS X includes a subsystem called "Classic" that allows almost all your old programs to keep running fine. However, to take advantage of Mac OS X's new features, you may need to update to "native" versions of your software.Check with the software publisher or with http://www.versiontracker.com/macosx to see if there is a free or paid update available.
Should I upgrade to Mac OS X today?
If you are a cautious person and your computer is doing everything you want it to, perhaps not. If you enjoy new things and like the new interface, or just want to be au courant, then go for it. There are also some great applications such as iPhoto, iCal, and iDVD that require Mac OS X. If you have an older, slower computer, like a beige G3 or early PowerBook G3, or any older PowerBook, you may be happier sticking with OS 9 (or buying a new Mac). Even if you have a faster computer, ...