You'll probably end up acquiring more than one SSD over time.
Large games I don't play off an HD, but I do keep one around with smaller games or stuff I'm not playing right this moment on it.
Currently using a 256 and a 180, plan to add another 256 soon.
As to OS drive, I'm not convinced an SSD is considerably better than a HD+SSD cache drive like the ReadyCache or whatnot. You end up with a lot of files that aren't actively used just wasting space on it.
Not to say it isn't nice, I'm just not sure it's necessary.
I'm a reasonable human being...at least once a year. Most other times I'm a catgirl.