I have also made this work on 2007 machines with x1900s.

Make sure you are using a TH2G Digital and also make sure you are using a Dual Link DVI cable to connect to the video card output and not a VGA.

Make sure it's a Dual Link and not a single Link. Also make sure it's not a DVI-I but a DVI-D cable. Something terrible that's not listed anywhere on the Matrox website is that if you accidentally use a single link DVI cable, your TH2G won't work right or if you use a DVI-I cable, the TH2G will default to VGA in instead of DVI in.

3 x 1280x720 or other 16:9 only work on the DVI connection, not a VGA.