I don't back up the OS, any programs, or drivers. I have all that on CD and is a cinch to re-install all of it. I always have done it this way so I am not dependent on OEM disks or 'restore' partitions on the HDD. What I do back up is all of the wife's files, her pictures, my pictures, the kids pictures (they take their own pics too), my movies, everyone's videos, my files, kids files. Comes to about 600gb worth in the end.
I alone have over 4,000 songs on my computer and about 150 movies in various formats but mostly in HD format for playback. One of these days I will get around to burning them onto DVD, LOL. When I do it should free up about 400gb worth of space, mostly on my computer.
As for restore partitions and OEM disks, if you use those you end up with your computer back to square one, complete with all those useless 'add-ons' and unwanted programs they ship your computer with.
For this reason, I always get a full copy of the OS it is running, then wipe the computers HDD clean with a full reformating (including removing the restore partition so the drive has one partition not two) and do a fresh and clean install of the OS. Then I can load it up only with the programs and freatures I want with none of the clutter that it came with. I have always found the computer runs both faster and more stable this way.