Sorry if this information has been discussed, but I didn't find it anywhere and have been searching for weeks.
I have been having issues with my ATI 2600s and 3650s (all pcie) for some time. I have searched around but until yesterday was unable to rectify the problems.
The issue was that when installing a 2600pro or 3650 into a system running 10.5.4. I would get graphic errors, ie disco screen, striped screens, other various errors which prevented me from using the cards.
What I did was:
1) Installed Leopard and upgraded to 10.5.4. I didn't upgrade to 10.5.5 because I was under the assumption that the drivers didn't work.
2) Installed drivers from here http://forum.insanelymac.com/index.php?showtopic=107526 thanks to LastExile.
3) Then repaired permissions.
4) Rebooted into safe mode
5) Using system prefs, I changed to my desired resolution.
6) Rebooted into a fully functional 10.5.4 system.

Then I decided I'd fool around with 10.5.5 some, so I backed up my systems.

7) Installed 10.5.5 update using Pacifist
8) rebooted with -f (got graphics errors again)
9) rebooted into safe mode again
10) reinstalled driver, repaired permissions
11) reset resolution again
12) rebooted into 10.5.5 fully functional

This has worked for me on 3 systems now
Two with 3650s and one with (2) 2600pros x-fired

The only issue I have had is my second 2600pro in the x-fire config shows 256m instead of 512.

I hope this can help someone using these cards, I am assuming it will work with others although I only have 2600pros and 3650s to test on.