Airport Extreme and os x 10.6.8 compatibility
The first thing to confirm is whether your iMac originally shipped with OS X 10.6.x Snow Leopard or whether you've already upgraded it from an earlier OS X version. If your iMac came directly from Apple with an OS X 10.6.x version preinstalled, then the good news is that your system is directly compatible with Mavericks. Mavericks is a free download from the App Store and your system already meets the minimum requirements.

That said, I would strongly suggest upgrading your memory to 8 GB or more RAM. Your system will perform much better than it will with 4 GB. This isn't absolutely mandatory, but the performance difference would be noticeable. To determine exactly what model you have and how much RAM it can support, I recommend downloading the free Mactracker app from the App Store. The DIY repair website, ifixit.com, has great written- and video tutorials on how to upgrade your memory.

The best/easiest way to upgrade … That's subjective. The easiest way is to download the Mavericks installer onto your Snow Leopard system and just run it. After making sure you have a Time Machine backup from which to restore in case anything goes wrong, of course. This is called an in-place upgrade and apart from the time spent downloading and installing, there is little for you to do. If everything goes smoothly, and it generally does, you'll have a running Mavericks system with all your apps and data intact. Note that if you're using any legacy PPC apps via Rosetta, they will no longer work and you'll need to find suitable replacements.

The best (again, subjective) way to upgrade would be more work, but would give you a pristine base installation, including the Recovery Partition on the drive, which enables you to command+r before the startup tone and enter a special startup mode that is similar to booting to an OS X Snow Leopard DVD. More information about Recovery can be found here:

http://support.apple.com/kb/HT4718

The specific steps I list below assumes that you have a working Time Machine backup that is current.


Purchase an 8GB USB Flash memory stick.
Download DiskMaker X and install it: http://liondiskmaker.com
Download Mavericks from the App Store, but do not run the installer.
Use DiskMaker X to create a bootable Mavericks USB installer.
Backup your Snow Leopard-based system one last time to Time Machine.
Boot to the USB image by invoking the Startup Manager: http://support.apple.com/kb/HT1310
Go into Disk Utility and format your Macintosh HD (or whatever your volume is called) - Erase - with (Mac os x Journaled).
Quit Disk Utility when it's finished erasing your Snow Leopard partition.
Select Install OS X from the OS X Utilities screen.
Go through the normal steps of installation.
During the install process, you will be offered the ability to either create a user account or to migrate a user account from backup; select the backup option and point it to the location of your Time Machine backup.
After profile-/apps migration has taken place, the system will log you in and you're done.


The latter is a lot more work, but rather than upgrading an existing system with its many years of possible dross, you get a pristine OS, replete with Recovery Partition, and all your migrated apps and data. This is my personal definition of best. Your mileage may vary.

If you have any questions, I'm happy to answer them.