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.
Fedex...for when you really need something to show up as slowly as possible.