I personally tested the follow :

One Mac pro 2.66 with 2 giga ram and a ATI RADEON XT 1900 4 sata drive amounted on the internal bays
One TRIPLE head 2 GO DIGITAL EDITION ( very important not the analog edition )
Catalyst m147

I was able to output 4 x 1280x1024 @ 60 HZ ( 4 = 3 on TH2G - 1 on second DVI output )