Arwyn Jones runs an excellent ground course at Cambridge. It's one day and uses real GNS430/530's hooked up to simulators which provide a pseudo GPS signal. It works exactly like the real thing in flight.
The day takes you from being a basic 'button-pusher' to being comfortable with the beast, including flight plans, approaches (for monitoring approved approaches, of course) modifying fligh plans in the air, dealing with deviations, and lots and lots of very practical tips.
It was probably the best day's training I've ever had and, as it essentially practical, I've had no trouble applying it in the air subsequently.
Arwyn's email is
[email protected]
AA