128GB will be 111GB after formatting and will be fine for Windows, programs and anywhere between 2-3 to 10-12 games depending on what games they are. Some games can be 20GB+ by themselves. Also remember for TRIM and garbage collection to work correctly you need to keep a minium of 20% of the drive free space.
I have had my 120GB SSD for just over 2 years now and just now am ready to upgrade mostly for space reasons. Even though with Steam and other digital distributation services you can very easily uninstall games you are not playing them reinstall as needed I get tired of having to do it everytime I get a new game. 256GB models have come down in price considerably and top tier Samsung 830/840 drives are in the ~$160 range on sale. I paid $240 for my 120GB Vertex 2 two years ago.
You can always easily add mechanical drives for media storage. I have between 2 and 5 mechanical drives in my PC at any one time for storage.