the below can be used as a guidance in terms of what you seek when you want to get a new SSDhttp://www.tomshardware.com/reviews/ssd-recommendation-benchmark,3269.html
dont put the SSD to an IDE interface as it sucks to the max in terms of performance. connect it to a SATA 3 interface for maximum performance.
get a separate SSD for your gaming only and leaving your HDD for stuff like backups. movies etc. this way you can guarantee that there is no bottleneck from the game data loading point of view. games like witcher 3/SC2 will take a huge time to load if you install it on a HDD while will take only a few seconds if installed in a SSD.
btw when you initially setting up the os in SSD do only perform a quick format.
also please make sure you have 8GB of RAM as it seems this is the current acceptable setup for most PC's when it comes to gaming.