Update to the latest version of nvclock from cvs (http://www.sf.net/projects/nvclock
modulename nvclock). On newer cards smartdimmer works slightly differently, it might work fine in the cvs version. Second I don't recommend to use smartdimmer it is based on an old nvclock and is less correct and compatible.
Second the correct syntax is either: nvclock -S 50, or if you want to do small increments for use with buttons: nvclock -S +10 for +10%.