this is how i got my new hd graphic card fully working...qe/ci enabled.
i've done a retail Leopard install 10.5.5 (using voodoo kernel) and after install i couldn't get rezolution change or qe/ci and in system profile my card was shown only with 256M. i knew that apple introduced suport for hd4870 starting with 10.5.7 so i've done an update....and suprize....black screen and white lines...the solution?
rebooted with -v -x and installed ATY.pkg from iAtkos v7 and got the card working flawlessly...full qe/ci...rez 1680x1050 for my 22" display....full hd movies work like a charm..even with quicktime, before a had ati x1600 and could only play 720p movies using PLEX, in quicktime 720p movies worked at around 10fps.
hope this helps