PRINT ME FOR REFERENCE.
I may have made a minor mistake or missed something but here's all the important stuff.
Steam and HDD:
Steam allows you to MOVE all of your game data between drives. There's instructions online, but basically it involves keeping ONLY the "Steamapps" folder and possibly steam.exe as well depending on whether you reinstall Steam.
https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129
I recommend you follow this rough guide:
1) update the motherboard BIOS if needed
2) backup any critical data (including DOCUMENTS folder to move back game save folders)
3) attach the SSD, then update it's firmware
4) Shut down PC, unhook hard drive (only SSD for drives now)
5) Boot into BIOS and make sure SATA is AHCI
6) Install WINDOWS 7/8 64-bit
7) Install drivers (MAIN CHIPSET driver from motherboard support site, then remaining motherboard/video)
8) Microsoft Updates, other programs
9) reattach hard drive, then boot to the BIOS
10) make sure the BOOT ORDER has the SSD first (or 2nd if DVD drive is first)
11) Make a new folder on the hard drive called "STEAM" (i.e. "E:/STEAM"), then MOVE the "Steamapps" folder and Steam.exe file there
12) DELETE all the other folders you don't need (make sure you got everything you needed); should just have the "STEAM" and "BACKUPS" folders left for now
13) Reinstall Steam (I honestly forget if just double-clicking "steam.exe" in the new folder does this for you, or whether you reinstall Steam then copy the files into that folder. As LONG as you don't delete the "steamapps" folder you can experiment.)
14) Backup Windows once satisfied (backup IMAGE of C-drive, the SSD, to the hard drive to RESTORE in case of problems)
Other:
1) overprovision the SSD (recommend Samsung Magician if Samsung SSD. Samsung 840 EVO recommended.)