You would do fine with a 256GB SSD. The OS will only take up about 20GB with drivers
and updates and games only benefit from being on an SSD when it comes to loading times, NOT during gameplay. You can have every program/application you use installed on an SSD and not make a dent in it. I have Win 7, all programs and 3 full games loaded on my SSD and am using less than half of my 128GB drive. I think you could be comfortable with a 256GB drive. You just need to remap or redirect your Users folder to a mechanical hard drive because pictures, videos, documents, etc don't benefit from being on an SSD at all.