Only thing I can think of are these 2 things:
Is the DVI cable you using between the TH2G Digital and the video card a Dual-Link DVI cable or just a Single-Link. TH2G Digital requires a Dual-Link DVI cable. Is the port out thet you are using the Dual-Link DVI port out for the card? Some cards have 1 Dual-Link DVI and one Single-Link DVI. Make sure it's the Dual-Link DVI port out. Only being able to lock small resolutions seems to point towards using the wrong port or wrong cable or both because the setup isn't getting the cable bandwidth required for proper resolutions.
I'm not understadning why you can't at least do 3840x1024 because any display should work at that with the TH2G. The TH2G Digital is only finicky with the 5040x1050 @ 57hz resolutions.
Your best bet for troubleshooting this would probably be here:
http://www.widescreengamingforum.com...forum.php?f=59