When they first came out, people were worried they would degrade if constantly overwritten, they still do, but much less of a problem. You can read from them forever, you can only write to an individual memory location so many thousands of times before it dies. As I say, not such a problem these days when the technology has improved, so it's merely a matter of space. You don't want to add extra games even though there is space, you want to leave room for Windows and GPU driver updates and the like. If you have Skyrim on SSD, and say New Vegas on HDD, they run the same, but you will get through doors and into caves a lot quicker with Skyrim, and of course your PC will run quieter and use less energy while you are playing it (less HDD noise). I would leave 80GB for system requirements, though anyone is free to correct me on this.