Confused, Need info

Why does logan suggest a 1 terribye hard drive AND an SSD? I know speed plays a big part of this, but I'm not sure how it works. What is the job of each one? I would like to know before I go out and buy them.

You install your OS on the SSD and put games and files on your HDD. A typical 128gb SSD is nowhere near large enough for your typical steam library. 

Nor is a 320 GB Hard drive

I have a 120GB SSD for my system and programs, and a 1TB hard drive for media - music, games, videos, pictures. If you don't have a lot of media, you can stick to just one SSD. And unless you don't mind spending ~$800 on a couple of SSD's for your media, a cheap 1TB hard drive is the best solution for media.

Thx