If you have the resolution set and still get mode not supported, make sure that its also set to 60hz for the refresh rate, if its not 60hz that can also cause mode not supported.
I have a custom built computer. The graphics card is a Nivida Geforce FX 5200 i have the updated drivers for it. I have a Samsung 32 HDTV model number LN32A330J1D. I have my computer hooked up to it with a normal computer monitor cable, RGB i believe is what it is called. The problem i have is my TV only supports a handfull of resolutions the highest being 1360 x 768. When ever the computer is hooked up to the TV and i restart it changes to 1280 x 1024 and the TV says mode not supported. So i have to get out my LCD monitor change the Rez setting back and then switch the cables and all is good. Now when the computer is pluged into the LCD monito which is a Starlogic widescreen HD20W and restarted it stays on the rez it was set at. What is happening when my computer is hooked up to the TV??