Start at Virtual Terrain Project for pointers on good representations of terrain and lots of discussion on making and displaying terrain.

Realize that you're probably going to have to have procedural terrain and/or repetitive blocks for the "generic" areas, with hand-modeling for areas that are important. Level of detail schemes will figure prominently in your work, as will (most likely) working from world-level layout fown to local areas.

If you're not set on creating your own engine, consider looking at one like Outerra ( Outerra ) that has solved the complicated display problems. There are plenty of complicated problems on the generating end, even with tools like World Machine ( http://www.world-machine.com/ ) and GeoControl ( Home ); these tools work well in the local terrain realm, but didn't do whole-planet all that well the last time I looked at them.