Yes theres a stopping point - for 1024x768 'Don't try to run more than 4 outputs with Catalyst' - true
Think we coud go round in circles here -
Theres no hard and fast rule. - except: test what you want to do before you need to do it!
I've had 6 x 800x600 running fine 1st go on an X1900XT in a 2007 MacPro running tiger. with 2 x TH2G
Also had 4 x 1024x768 from 2 x DH2G on the same spec...
TEST , try, test and make sure you can repeat it....
Simon