OS, Adobe, and steam should be on the SSD then. the problem is that steam games are large and you will be in a cycle of moving game data to and fro the HDD to see any benefit in gaming. you could install steam and the games to the hard drive but the games will be slow then.
I would find the slowest games and load them to the SSD, the less often used games can live on the hard drive.
for the SSD to be beneficial the data needs to live there.
Fancycache (stupid name for an awesome program) would be my suggestion, use a SSD as a dedicated cache and everything will be sped up, on the fly, with no input from you. look into it.
I would suggest a 120GB ssd and the largest HDD you can afford use fancy cache and they both are now a hybrid hard drive as far as windows in concerned.
may not be applicable on your OS, you never said which you have.
I have used both and SSD caching is by far the easiest.
if you have an intel based board you may have SRT available to you. SRT is intels implementation of SSD caching. no fancycache needed if the board supports it. intel does limit SSD cache to 64GB, fancycahe will let you use entire SSD and extra RAM as cache.