Steam has a page outlining what you need to do for transferring the game library without redownloading everything.
https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129
As for the drive with your OS. You're going to need to format that drive and do a reinstall for the new rig (assuming its a new motherboard). There are ways to kludge it depending on the OS, but they tend to make for unstable builds. Also you'll need to make sure your OS license allows for transferring between motherboards (retail does, OEM generally does not). If its an OEM license, you can call up Microsoft when you get to the activation part and talk to a support person. Sometimes they'll just give you a reactivation code. Sometimes they'll wants some cash for a new code. Sometimes they'll tell you you need to repurchase Windows. I've had all 3 happen depending on the person, the mood, and where the stars are aligned.
As for the non OS drives. If they're just storage drives, then you shouldn't need to reformat them unless you want to. If they were drives with programs installed on them, you'll need to reinstall that software when you reinstall Windows.