Be sure to completely erase the older ATI video drivers by using driver sweeper/cleaner (install driver sweeper, uninstall drivers normally, boot into safe mode, run driver sweeper, reboot normally, install 10.4a).
This actually appears to have worked! I'm not sure if there was some issue w/ leftover drivers that driver sweeper fixed, or 10.4a was what helped. Either way I'm happy.
Guess I'll experiment with newer drivers sometime in the future (being sure to use driver sweeper first), but for now I'm happy.