Bernie Sanders elected, what will he accomplish?

[Mehmentum]

He will accomplish virtually everything there is to be accomplished (given a Dem congress, of course).

Why on earth would you assume that?  Republicans will control the House (at least until 2022, after redistricting), 99% guarantee of this.

As for the OP, given an at least partially Republican controlled congress, the vast majority of Bernie's campaign promises are unlikely to happen.  The only things I could see any Democrat achieving legislatively are:
-Prison/Drug Reform
-Immigration Reform (maybe)
-Tax Reform

And keep in mind any bill that passes the House will necessarily have earned John Boehner's approval, which greatly restricts the actual content of these bills.

Of course, there are other things a president can do... appointments (particularly to the Supreme Court), executive orders, and foreign policy.

[Mehmentum]

This is why I'm still planning to vote for Hillary.

Which would accomplish more of the Democrats' agenda, Hillary with a Republican Congress or Sanders with a Republican Congress?

The answer is definitely Hillary. Sanders and Hillary's agendas aren't that different. Hillary would be able to accomplish more of it than Sanders.

No, Hillary with a Democratic Congress would accomplish more than Sanders with a Republican one.  

The idea that the GOP would just go along with Hillary's agenda is laughable.  It would take a lot of wheeling-and-dealing for her to get anything done, let alone her most progressive reforms.

Well, imo Clinton would be better suited to do the 'wheeling and dealing' necessary to get anything done.  I don't think anybody is under any illusions about how much can get done on either side without the Presidency, House, and a filibuster-proof Senate majority, however there are some areas where some progress could be made.  In addition, I think Clinton would be better able to avoid debt ceiling and government shutdown crises.