I don't need much more than 100GB for work related softwares and files, since I moved all my work related video/photos library to an external storage. There is like 50GB of iTunes videos/movies that I constant delete/download.
350GB of free space to use for games and other stuff, as long as I don't change my mind and bring my video library back, which I think would be unnecessary. The problem is, Bioshock Infinite alone takes 32GB.
Don't think I'm condescending (even if it sounds like I am), but the basic rule of thumb is:
Fast drive for the OS, applications, and games, since they need the speed.
Slower drives for files, movies, and music, since they don't.
Like you fire up a game from an SSD, and you'll notice shorter load times across the board. This is obviously something you want.
But movies and music? Even high def Blu-Ray rips can fire off and seek from a old USB 2.0 thumb drive near instantaneously. They don't need that speed. What they do need is space, because they rack up.
Since I'm guessing you have 512GB on your SSD, that should be more than enough for your OSes, games, and apps. Yeah, games are huge these days, but that can be mitigated a bit with a fast internet connection. Keep them around when you're playing them, delete them when you're not.
Though really, the best thing to do would be to shunt your older games to the USB HDDs for storage, and keep the current ones on your SSD. Then, when you want to play them again, just move them back into your Steam program folder. That way, they're always handy.