Obviously if your Mac came with leopard then your reading the wrong guide.
This guide is to help those Panther, Tiger users who wish to upgrade without problems. It's short, simple (like the company name) and straight to the point.

WHY: Well if you toss in the DVD and choose upgrade your in for a HEADACHE! As of this writing the retail version upgrade is flawed! Sorry, Apple we love you but you have got to fix this issue! It's not 100% flawed but if your Mac is over 4months of use on Panther or Tiger then DO NOT UPGRADE through the disc. Archive and install is your second option, we'll discuss the correct way.

Archive and Install is an OK choice if you have an iMac/MacPro. This option will compile your 10.x system and place it in a different area. You'll still be able to boot into it as well choosing the startup disc then your old OS. HOWEVER you still may have some issues. Issues like blank screen, frozen mouse, laggard key strokes, delayed app opening and worse yet App freeze.

The correct way! After doing more than 20 upgrades on 3 different machines (yes, we wasted alot of time and we hope you benefit from this) testing this/that and the other we came to one conclusion: Erase and Install BUT only after you have made a complete clone of your existing drive.

Leopard has Time Machine, a backup scenerio that creates archives of your files as you change them. It works flawlessly when it's setup right and done with an erase and install. You need an external drive for this (Firewire 400 is marginally faster than USB 2.0 after initial setup, FW800 is the sure ticket if you can do it). BEFORE you buy any old HD take into account how much data you have. If you have a 160gb HD and it's 50% full get a 320gb drive. Basically get double the HD space if you can afford it.

Now you should start to get a picture. Your Mac is running Tiger/Panther. You have a new shiny external drive. You maybe bought Mac Pack 2 and in that case also have Cloning software. STOP! Open disk utility, and create a partition scheme on the drive (2 partitions). It will create two equal partitions (remember we said double the HD size for your ext?). Label one Backup, label the other Time Machine. Ah Ha!

Clone your main drive to the Backup partition. This will take anywhere from 1-4hrs. It depends on HD size, Ram, and of course CPU. After it's complete double check your backup and make sure your docs, apps, and all other information is there. Stuff you care about, don't worry about the system files too much.

Ok. Now you can slide in Leopard and reboot. Leave the external drive connected.

Once you get past the language screen goto Utilities - Disk Utility. Erase your main internal drive you plan to use OR just the partition.

Exit out of disk utility and continue with the installation. After the installer reboots and you get to Migration Assistant this is where you WANT TO TRANSFER FROM ANOTHER PARTITION on this Mac. Select your backup drive as the unit to transfer from.

Done. Your users, files and even applications will be transfered. Please double check your applications to insure they function OK. You may need to download an update. I trust you already checked to make sure they were compatible... Or at least updated each one, Open the app and click on the App name from the drop down and select check for updates. This may also be hidden under Help.

You now also have a dual boot system (FW only on PPC based Macs, Intels can boot from USB) and a separate partition for Time Machine. If you feel all your information has transfered succesfully you can go ahead and re-partition the external for one large HD or 2,3,5,20 whatever you need. But keep at least your HD's size for Time Machine. More is always better. I believe we saw a reco for 2-3x's your existing HD for Time Machine if you do A LOT of work especially with large files such as Photo or Video.

Tis all, happy holidaze and a satisfactory new year.