Because of the gfx upgrades that were done, the game requires more memory for its textures when the setting is on high. On windows XP, 2GB is enough, but vista & windows 7 take up so much memory on their own that you need at least 3GB to be able to load the textures. The game prevents you from putting the textures to high as a safety measure when it detects insufficient memory, because we know that it will cause problems at a certain point (really nasty ones).
