D3DXSprites are just a more easy way to draw textured quads using screencoords. There are modules allowing you to draw textured polys/quads (using your own UV coords so you can use subrects on a texture). So no connection to point sprites there.
If in some cases OpenGL is faster than DX then it might just be because it's better batching the single draw calls which is what BMax does pretty bad in terms of driver calls (batching your quads using one of the texture mods should be a no brainer).
But to be honest - the game you want to do in such a low resolution would be easy to do using softwarerendering on a >= 200-300Mhz CPU I'm sure you won't be limited by whatever language/way to draw/engine you use so I guess you'll be happy coding your game after that the weekend ;-)
If in some cases OpenGL is faster than DX then it might just be because it's better batching the single draw calls which is what BMax does pretty bad in terms of driver calls (batching your quads using one of the texture mods should be a no brainer).
But to be honest - the game you want to do in such a low resolution would be easy to do using softwarerendering on a >= 200-300Mhz CPU I'm sure you won't be limited by whatever language/way to draw/engine you use so I guess you'll be happy coding your game after that the weekend ;-)