Re: 7600GS at 2560x1600
I'm honestly not sure why you're seeing single link TMDS with the 7600GT card. When I attach a Dell LP3065 panel to a 7600GT, I get the full 2560x1600 resolution when using the 1.0-9746 driver.
I suppose it could be the VBIOS on the graphics card you're using. You might want to contact Gigabyte and ask them to confirm that the card you've purchased has been verified to work with the Dell LP3065 panel.
In some very rare cases I've seen a bad motherboard BIOS cause strange behavior like this. Have you verified that you're using the latest BIOS?