Fast capture of screen data into an array of RGB values

I need to capture RGB data of a portion of the screen
as fast as possible (capturing for animation purposes).
My current method works, but I've seen other programs which
are faster at capturing animations of the desktop.

I eventually need the data in RGB(888) format, but I could manually
convert the data from the current screen format (e.g. RGB555).  
If it could speed up the whole process, then that's what matters
most.

Here is an outline of my current method:
  captureHWND = myGetSelectedWindow(); //This could be any window (even the desktop itself)
  dataBuffer  = getDataBuffer(); // allocate a place to put the desired data
  bitmapInfo = myGetBitmapInfo(); //Initializes a BITMAPINFO structure (with biCompression = BI_RGB)

  captureHDC=::GetWindowDC(captureHWND);
  bitmapHDC =  CreateCompatibleDC(captureHDC);

  bitmapH=CreateCompatibleBitmap(captureHDC, bitmapResX, bitmapResY); //resolution relates to area of interest
  SelectObject(bitmapHDC, bitmapH);

  do //loop through capturing multiple frames
  {
    GetDIBits(bitmapHDC, bitmapH, 0 , bitmapResY, dataBuffer, bitmapInfo, DIB_RGB_COLORS);
    //do work on each frame of captured data
  } while (...)

Perhaps there is a lower level method of getting at the screen data or an
approach which requires less work out of Windows.  It does need to work
on all of the basic windows platforms: Windows 9x/Me and WinNT/2K/XP

What I want: A sample code snippet that puts RGB screen data into an array
based on being given a specified rectangle of the screen and is faster than the
method I currently use.
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