Well, Depending on the amount of software you think you need to install, even a 120GByte SSD will do. It did in my case. I upgraded a 7-year old system at that time with a 120GByte SSD (Samsung Evo 840) as a boot drive. It was like working on a new computer again.
While the comment from ConstanceJill is a good and valid one, given the specs of your machine, I assume you are not that avid of a gamer. Well, not the latest games that require huge texture files to load at an instant. In that case you can easily keep them on your standard spinning disk. Especially if that is a more modern one that comes with a lot of Cache memory. Thing is, by separating Windows from your games or data, the content on the spinning hard disk will not fragment that much and after a while the Windows file-system has optimized the content of that disk anyway.
Fragmenting is/was a big performance killer with spinning disks. But as you keep that now separate, You will find that by spending less on your SSD, you have more money left for other parts, like bigger/better videocard, a faster or bigger monitor. Or simply a second monitor (if you did not have such a setup already). Or the money saved can be used to start saving up for a new computer, tablet etc.