Put a 640x480 image in the top left corner of a 1024x512 texture and adjust the UV coordinates of your quad so that it only shows the 640x480 section.
Crap. I understand the principle, but it seems like a waste of resources (probably because I don't understand why loaded 640x480 texture will be bigger anyway - all this 3d stuff is so confusing.) I want to make a 3d game, but all this new stuff makes me want to come back to 2d and I don't want to do that (that's why I didn't make any games in two years!).
I'll probably do it differently then - I'll cut out items as separate sprites and make real 3d locations. So the game will be true 3d with sprites and not just faked-2d.