If it were my system I'd shoot for the 128GB ballpark. It's plenty for the OS alone and would even store some games/programs if you wanted to. Heck even on my WD Black 1TB (my main C: drive) that I have now I'm only using 168GB. I store more stuff on my other drive but I'd say that 128GB would be plenty big for a SSD as long as you didn't put absolutely everything on it. Besides, get too much bigger than 128GB and you start talking some serious $$.