I specifically chose an ATI HD5770 card because I hate recompiling the Linux kernel every time I want to use an nVidia card. I stuck the HD5770 in my system and booted to Red Hat Enterprise Linux v 5.5 (64 bit) and it seemed to pick the drivers right up. I didn't have to do anything fancy to get the card to display under Linux.
Now I can't get the 1920 resolution I can under Windows but the HD5770 will go to 1600x 1200 under RH Linux. I don't know about loading the Catalyst toolkit under Linux as I've never even seen Catalyst files for a Linux distro. However I can say that my HD5770 works well under RH Linux. I do have a duel boot system and the HD5770 ALSO works under Windows XP 32 bit.