Mac OS X Leopard (10.5) is due for a release in just about a month or so from now and some members might have some questions on moving from Tiger to Leopard once they purchase the latest OS from Apple. (The following suggestions might work for Mac OS X 10.3.x Panther users as well but I would personally feel you'd be safer if you followed the last upgrade option.)

While I haven't used Leopard yet, the installation options should be the same as for someone that went from Mac OS X Panther (10.3.x) to Tiger (10.4) i.e. you will be able to upgrade your Mac's OS in three ways. Here is how Apple described the Tiger upgrade options:

Three ways to install

When you start the upgrade process, you’ll get to choose one of three ways to install Tiger on your Mac.

- Simple Upgrade: This option upgrades all the important components of your old system software while preserving all your personal files and most settings. Could it be any easier?

- Archive and Install: This option saves your existing system files in a special archive, then installs Tiger. You can choose to automatically import your user information and settings into Tiger so you don’t have to reconfigure your printers, Mail and other applications.

- Erase and Install: This method completely erases the destination volume, then installs a new copy of Mac OS X. You’ll have to copy all your files and re-enter all your account settings.
The Simple Upgrade option to my knowledge is the least reliable as I have heard of people going first with this option only to encounter some quirky behaviour from their Mac farther down the road and finally going for a backup of all their files on an external hard drive and doing an Erase and Install to correct the problem. If you are upgrading from an OS X version older than Mac OS X Tiger (10.4.x), I would advise against this option, though.

The Archive and Install is ok in a lot of cases unless your previous Tiger install had problems: you could then be importing, in the brand new OS install, those problems using this method because it'll most likely keep those software conflicts in the new system. Again, if you are upgrading from an OS X version older than Mac OS X Tiger (10.4.x), I would personally not suggest this option.

The Erase and Install option is by far my favourite: I use an external hard drive to back up regularly my data, so to me erasing the internal hard drive is no biggie. All my files are on the external HD and photos I take are burned to DVDs as well. This install method wipes your drive clean, defrags it in the process and you are basically starting on a fresh new page! This is good for any Mac OS X version you might be upgrading from.

For more info on these different types of installs, refer to Apple's Knowledge Base Article Mac OS X 10.4: About installation options.

Now about the installation itself!

1- Just insert the DVD in the optical drive

2- Restart your Mac while holding down the C key

3- You will be guided through the installation procedure and be prompted at one point to choose which install option you want

4- Once the Leopard install is completed the Mac will restart itself

5- You will probably be asked to register with Apple upon restart (which you can do or not BTW)

6- Enjoy your Leopard Mac!

Hope this helps!