To your first Q are they worth it? YES definitely imo.
As to how much, well get what you can afford. I would suggest at least 60Gb as although you say you will only use it for the OS I guarantee you will want to put all your Apps/Games on it once you've tasted the speed. If you can get a 128Gb Samsung they are prob the best value for money atm, although I think the Corsair is basically a rebadged Samsung - maybe others can help more regarding those as I am more into the OCZ Vertex series.