I did some looking and decided you were right so i added a 80gb ssd just for my operating system. I also changed the psu to the same line of psu's but upgraded to the 1000w version and changed the video cards to the gtx 580 classified in sli
For just an OS, 30 to 40GB is fine.
I would preferable go for a 60GB SSD.
It will fit your OS, along with other data.
What is your entire budget?
There are many changes you should make if you are getting a pair of high-end cards.