As someone who has a SSD as a boot drive and a 2TB WD green for everything else, including games. I can say that, YES it will affect your gaming.
I thought about going this route as well but decided against it as the poster can attest to. Ideally everything stored on an SSD is ideal but not realistic to most peoples budget scenario. I would recommend a smaller SSD as a boot drive and any drive labelled as "black", any velociraptor drive or if your budget is really crunched going with at least the blue series drives as their data IO is usually much better than the green drives. "green" drives usually work by using a lower spindle speed and when they are not actively reading or writing by spinning completely down and going into a sort of standy mode both of which negatively impact the higher needed disk IO for gaming or other HDD intensive tasks