If I understood right, you will want to CreateOffscreenPlainSurface()
with D3DPOOL_DEFAULT
flag, so driver will choose appropriate memory location automatically. But it never guaranteed that it always will be videocard's on-board memory.
By the way, premature optimization is the root of all evil. =)
Edit: Another option is to switch API from DirectX 9 (which is obsolete) to DirectX 11, which allows much more precise resources manipulations. Also, OpenGL goes somewhere in between. Both, is a huge code rewrite.