Upgrade to Yosemite question
I never do upgrades any more - I made this from several times from Snow Leopard to MountainLion and my MacBook Pro 15" Dec 2010 got sloppier and sloppier in performance. Instead, I would recommend the following procedure which gets you a real fast Yosemite installation with a fresh and clean system:


Make a full backup with Time Machine
VERIFY the Backup of your most important data, not your applications! That means also a separate Backup of your Library folder!
Copy the most valued data onto another USB Drive for faster access.
Look for all the Yosemite updated drivers for your peripheral devices, like mice, headsets, special keyboards, USB video- and soundcards etc. Save the installers on the second USB Drive where you copied your most valued data on to. Do this also with your favourite OpenSource Software like Firefox, Chrome and your application installers etc.
Download the Yosemite installer.
Create an installation USB stick.
When booting from the USB stick running the installer, delete and format and re-partition your harddisk you want to install on.
Install Yosemite
After installation, configure your new system as you like it, play around with it to get acquainted.
Start installing drivers and tools and applications. 
Start copying back your most valued data and your backed up
Have fun. :) 


You will get a system exactly for your needs, without any old drivers/kext and you cleaned up in great detail. :) 

My MBP Pro is now flying at high speed below radar... :)