Aha. Very interesting.
I guess then my real question would be, what are games like Neverwinter Nights doing when they offer "16 bit texture packs" or "32 bit texture packs". I always go with the biggest number myself, but I have no idea what the real difference is. Perhaps in a game that size it's really just the source graphic they're talking about, as the lower bit rate might make hard disk usage lower.
Of course, I wouldn't expect you to actually know the answer to that. I suppose you're as qualified to make a guess as I am though.
But the important info for me is that I can just load up my 32-bit source graphics and set the Display to something lower, and OpenGL will just figure it out. That's certainly a relief.