It really depends on what you want to do?

If you are planning only to store the signed documents, it is not very different
from unsigned documents.

We also sign web-based infopath forms (at least partially) using Infopath
services on MOSS.

MOSS also comes with collect digital  signature workflow if you have professional
version of the ofiice 2007.

hi,
It works on 2003 and 2007,you should use migration tool..

http://www.arx.com