Here is a link that might help you out
I personally haven't done this yet, nor have I really considered it. Now that I'm thinking about it, I would probably only change the rear for fear I might make the car too stiff if I upgrade the front also. People who have changed out the rear are saying good things about the difference they feel
http://www.lexusf.com/forums/showthread.php?t=775&highlight=sway
http://www.lexusf.com/forums/showthread.php?t=775&highlight=sway