I have a 20" iMac Aluminium MA877 with HD2600 PRO. Just today I update it with new ATI MXM II HD4650 1Gb from Acer (http://cgi.ebay.com/ATI-HD-4650-MXM-II-Vedio-VGA-Card-Acer-VG-M9606-005-1GB-/260556752817?pt=PCC_Video_TV_Cards&hash=item3caa6487b1) card. Physically it fits very well, the PCB design is pretty much the same. But unfortunately after power on the Mac I've got the black screen.
Googling I found that may be I need the EFI update for this card.
Could someone help me to make it work?