Stealing Artifacts

Hi,
   Anybody out there have any idea how good an Agent must be in order
to have a good chance of Stealing Artifacts? Do skill ranks of the
target play any part in this & if so how much?
   I'm interested because I'm considering trying to nick an Elven
ring from Gandalf but don't want that "terrible old man" to kill one
of my few decent agents ...

Ian Harris