if you update to leopard, it just brings all of your apps and any of the 'behind the scenes' architecture of the OS up to speed. you don't lose any apps, they're only improved, and in most cases run faster than smoother than they did before. software update makes minor improvements and bug fixes in apps and the os, while buying a new OS will make major updates to your apps, add new apps and features, and obviously also enhances the overall performance of your apple