Upgrading to 2.0 with "the least amount of effort" depends on your
setup and your personal judgment.  E.g., is rebuilding a ton of
packages from pkgsrc "effort" or "no effort"?

The less customization that you have on a system, the easier it
will be.

I've upgraded systems usually in one of these ways:

 * Backup /root, /etc., and a couple of others; do a full
   wipe-and-install of the OS as if the disk were clean;
   restore backups (merging /etc with the new /etc with some
   care).  Now go to pkgsrc and reinstall any applications
   that you had added under 1.6.x.  (^&

 * Use the sysinst tool's "upgrade" option, rather than
   overwriting.  (Still it's a good idea to have backups.
   And don't forget that *upgrading* will remove the rc.conf
   line, "rc_configured=YES", which a fresh install puts in
   for you.  (I suppose that the rationale is that a fresh
   install is fairly secure, and uses components that the
   installers knew about.  An update of an existing system
   has components installed and features enabled that the
   installer may not be able to account for...)

 * Use the system source tree's build.sh to do an install over
   an existing system.  (I've done this for a couple of machines
   tracking -current.  I have not done it for a "normal" system
   update.)

If you have a spare disk, another option is to install the spare
disk as your primary, install the OS to that disk, then merge /etc,
and such, from the old disk.

I don't represent that any of those is necessarily the least effort
for any particular situation.  But all are generally fairly
straightforward.

(Once 2.0 is released, I'll probably be upgrading several systems,
but I'm holding off for now.)

Good luck.

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/