First of all, 120gb is really too small for a C drive.
Many things default to the C drive.
If it fills up, you will lose performance and endurance.
With lower ssd prices, see if you can't manage 240gb.
You may have another option.
If you buy a Samsung ssd, they have a free ssd migration aid that will move your windows C installation to one of their ssd devices.
The migration aid has a provision to exclude folders.
Large data folders like videos are simple to exclude.
After the migration, such folders will be available on the old hard drive.
Unfortunately, apps that were installed using the registry will no longer work if you excluded them.
I have no experience with Steam and how that might work.
Here is a link to the manual.
http://www.samsung.com/semiconductor/minisite/ssd/downloads/software/Samsung_SSD_Data_Migration_User_Manual_ENG_v3_1.pdf