Pointless to just use Os all you get is faster boot time. You really need your apps on there also. I run an 80G Intel one and after 1 game and my few apps I have 23G left, I wouldn't get less then 80G.
Apps are faster, even if you only have your OS on the SSD. That's because the SSD will take care of all OS related stuff, leaving your application drive free from any OS calls.