Efficient Multiple Renders to Texture

I’m beginning a project and am developing on a win32 P4 with 512MB RAM and a FX Go5600 chip. I would like to try the procedure below. Please let me know if it’s at least feasible.

  1. Render a scene to Texture (pbuffer)
    a. copy both the color and depth buffers to separate textures.

  2. Clear the pbuffer

  3. Render a single object to Texture
    a. Use a shader to store info based on comparisons of the above two textures and this single object.

All of the textures will be the same size (and pixel format?) and none will be displayed. I hope to perform this process every frame, optimally, or at least twice a second, acceptably.

Is there an optimal way to copy textures from a pbuffer to a texture? If possible, I’d like to avoid vendor-specific extensions where possible, though I understand that I need WGL_* to do both pBuffers and Render to Texture.

Thanks in Advance.