Below is a basic layout of my storage usage on my 2TB Hitachi Deskstar.
279GB = All program files
92 = Program files minus Steam
500GB = Other files (videos, pictures, etc)
Unfortunately, my current hard drive runs at only 5900 rpm, terrible for a boot drive. So, I've decided to bite the bullet and get one of those fancy SSDs everyone's been all the fuss about. However, the most I can afford is a 120GB OCZ Agility 3 for $120. I'm probably going to put all my programs other than Steam on the SSD and just wait a little longer for level load times. I'll also leave all my other files on the mechanical hard drive. Is this a good idea? And is it possible for me to put only certain steam games on the SSD as well? TF2 level loading times are pretty annoying.
You can use an SSD as a boot (system) hard drive and keep all of your other data on the HDD - 120GB is more than enough to store windows (and updates) and provide a loading buffer for your programs from the HDD which will make your games load faster but will not be permanently stored on the SSD - kinda cool.