Why do I have THIS http://www.kuenzle-family.ch/philippe/error.jpg when two object are overlapping using a GForce2 MX and I don''t when I am using a GForce3... Also, I saw some demo running open GL on my GForce2 MX not having this problem. The "sea" was one huge polygone and "mountain" was coming out of those and the border between mountain and sea wasn''t doing that mess... Thanks

That error is commonly refered to as "Z-Fighting". It can be resolved one of two ways:

A. Increase your Z-Buffer depth.
B. Change the Z positions of the two overlapping surfaces so they don''t come that close to eachother.

Try A first if that doesn''t work try B.