I am assuming you really do have an ADI GD-790 monitor ?
When you press Crtl-alt-+ I am assuming you get a desktop at 640x480, repeated pressings of ctrl alt + (or -) will take you up (and down) the modeline for your default colour depth. When you do this is 6480x480 the only resolution that works ?
I would say that your sync and refresh is going out of the range your monitor can cope with at the first resolution in your mode line (which is the one X will start as)
These should have been auto detected but depending on how you set up X maybe that didn't happen. A quick google suggests that the following settings might work for your screen.
Find "Section "Monitor"" and add these before the next "EndSection" HorizSync 30.0 - 110.0 VertRefresh 50.0 - 160.0
(noting the case)
Either that or there is something going horribly wrong with the nv driver.
Caveat- Manually specifying the wrong sync and refresh rates can actually damage CRT screens. *most* modern CRT's will in every case do what yours is doing and show a message rather than obey incorrect figures, others won't. So if you have the user manual for your screen to hand then I would double check the settings I have given you before applying them (you should find them in the specifications section) If your monitor makes any squealing or ticking noises or shows a dim or folded over picture then turn it off immediately.