Hardware surface

I have upgraded to Win2000, with DirectX 8, and now my video card (Matrox Mystique, 8Mb) reports its
capabilities correctly:

(Set 1024x768x16 video mode)
Video driver: directx
Hardware surfaces are available
Window manager is available
Hardware to hardware blits are accelerated
Hardware to hardware colorkey blits are NOT accelerated
Hardware to hardware alpha blits are NOT accelerated
Software to hardware blits are NOT accelerated
Software to hardware colorkey blits are NOT accelerated
Software to hardware alpha blits are NOT accelerated
Color fills are accelerated
Video memory: 5815 Kb

My understanding (please correct me if I’m wrong) is that I should be able to use
SDL_HWSURFACE|SDL_DOUBLEBUF and get a speed increase. But these flags don’t work with
SDL_FULLSCREEN. The error message I get is:

Couldn’t set a 1024x768 video mode: DirectDrawSurface3::lock: DirectDraw is still drawing

Without SDL_FULLSCREEN I get no error, but the speed is the same as using SDL_SWSURFACE.

Is there an explanation for this?

thanks
Gib