The key is to find out what information you require during which situation of flight. This is an iterative process and changes over time as you gain xc experience. Start simple!

My Setup is the following:

I like XCSoar's pages concept therefore i usually configure 2 Pages,
* One that is Context aware, the "auto" page
* One that contains secondary information i deem important.

The first page contains always these info boxes in all three modes:
* GPS Height
* Speed over Ground
* MCCready

In circling mode I use additionally:
* Thermal Gain
* Thermal 30 Second Average

In cruise mode the following extra boxes:
* Direction to Waypoint
* Distance to Waypoint
* Height Required to Waypoint

In FinalGlide Mode:
* Distance to Final
* GlideRatio
* Wind Component
* Direction to Final

The Auxillary Page includes the following:
* UTC
* LocalTime
* FlightTime
* Distance to Home

I don't do competitions (yet) and am currently the causal xc pilot type. But I assume there are more tools that you could configure to be useful during comps.

Hope that helps.
- Folken