By default, most OpenGL drivers will use an LRU (Least Recently Used) algorithm to swap textures.
taken from Understanding and Using OpenGL Texture Objects So, if this was me asking the question, I would be happy that Mark's implementation of DX driver uses the Automatic Texture Management DX provides and that OGL uses a similar LRU algo.
Anyway, other than getting a response from Mark and/or posting on the API forums you might want to do a search for Video Memory monitors and see what happens.