TH2G usually only works up to 2400x600 (is 3x 800x600) on a MDP-DVI single link (£20) adaptor
I've just purchased a MDP to DVI-D adaptor - so will let you know my findings with a standard TH2G-D (DVI) (Monday)
Matrox also have a MDP version of TH2G - I don't have one of these to test, but the specification seems to suggest higher resolutions:
http://www.matrox.com/graphics/en/products/gxm/th2go/
Simon