If you’re running an Intel-based Mac you should really run out and buy the latest version of Apple’s operating system, Mac OS X 10.6 Snow Leopard.
To learn more, see Apple’s Snow Leopard pages and MacWorld’s extensive coverage. To check that your critical applications will run under the new OS go to the Snow Leopard Compatibility page on snowleopard.wikidot.com.
Although Snow Leopard provides very few new features, it consolidates all the work Apple has done since moving to Mac OS X in 2002. By removing Power PC code and optimising Intel code, the installation is smaller than 10.5 and runs faster. And at only £25 for a single user and £40 for the Family Pack to upgrade from Mac OS X 10.5 Leopard, it shouldn’t be a difficult decision.
If you’re still running Mac OS X 10.4 Tiger your best bet is to buy the Mac Box Set. For £132 you get Snow Leopard, iLife ’09 (iMovie ’09, iPhoto ’09, GarageBand ’09, iWeb ’09, and iDVD) and iWork ’09, Apple’s productivity suite which includes Pages ’09, Numbers ’09, and Keynote ’09.