the TX850 will probably be near its limit here, but that can be a good thing, since it will be more efficient at idle then.
If the things you write fits your budget, I'd say go for it.
Only thing is to pick a gen 3 motherboard (asrock extreme3 gen3 for instance)
I would actually consider putting the OS on the HDD and then putting your favorite games on the SSD.
1st of all, the size of a game is somewhat fixed. The OS will grow larger as you use it, and per default, documents and downloads will go on the same drive.
2nd, As a gamer you often care more about loading times in the game than how fast your computer turns on. Your call of cause.