I never did find out the many fglrx options for xorg.conf, so I really wouldn't know. The only options I have played with are VideoOverlay, OpenGLOverlay, and OffScreenPixmaps. I found that using VideoOverlay (Xv) has a much lower CPU usage rating than OpenGLOverlay (OGL rendering). OffScreenPixmaps was needed in earlier driver releases because of a screen corruption bug, but it has been fixed since the 8.01 release.
I have found that "flickering" can mean a lot of things for different people. Some call flickering what I would describe as image tearing. What I call flickering is when the image on the screen comes on and off in rapid succession. This flickering I only experience when attempting to render videos in OGL or Xv while Compiz is on, which is more of a X issue that a driver "bug".
For my HD2900XT I just use the overlay options I mentioned above, and the card seems to perform adequately just like that.
Just as a further point, for anyone else in the same position, I have worked around the flickering video problem by using the Compiz video plugin and compiling mplayer from SVN to enable support for it.