I am aware of ATi cards not doing the higher TH2Go resolutions, although MarkB the Matrox Rep can tell you more.
The compatibility tool is here:
http://www.matrox.com/graphics/en/support/drivers/download/gxm/tool/
But I dunno whether it'll tell you if the 4870 will do 3840x1024. I swear Matrox used to actually list this info on their site, but I can't find it. :(
Yes I did find that site. But can report that on my new installation the HD4780X2 supports 3840 x 1024 and runs fine on a TH2G with three monitors.
Re W