OK I'm a bit out of touch, but in 3D games the CPU does all the maths and rotations etc of the shapes in 3D and then sends the coords to the 3D card to render right. The 3D card doesn't do any fancy maths does it? Or am I wrong here?
It depends. You used to be right here, because IIRC Max2D did not enable TNL ( hardware transform and lighting ) by default, but I think it does now. Or if it doesn't, it's available with a third party module. Pretty much any 3d card sold in the past decade can do it, so there's no sense not using it.
As for the 3d thingy, can't you calculate the projection manually and use Tim's textured poly module to do the drawing? Basic 3d projection is very simple, so it won't take any time. I'm guessing that both GL and DX modules lock you into 2d drawing with a 2d projection matrix so switching to a 3d projection matrix, drawing a quad and then switching back to the 2d projection matrix again would be slow because of the time it takes to reset the PM. Might as well stick with 2d and calculate the projection yourself, I'd say.
EDIT: Just to be clear, I've not used Tim's module, so I'm not sure how it works. It may not be suitable, so if I sound wrong here, I probably am.