You can't tell how much video memory is used. Especially not when we're 3D accelerated like in BMax. I don't think never versions of DX have this command. Textures might get converted, swizzled, bank aligned and more by the driver/hardware so it's really impossible to tell. You could try to create a bunch of proxy textures using the corrosponing GL command to get a rough estimate. But those days are over, where you could get the amount of free video memory :)