how to increase screen resolution to 1680 X 1080 in Ubuntu 10.04 LTS

I have an emachines ET1336-03W PC. I have it partitioned so I can boot into Ubuntu 10.04 LTS or Windows 7. I have an AMD XFX graphics card in the pc which has two ports. one Port connects to the Samsung 22" LED monitor which has 1680 X 1050 native resolution, using the computer monitor's cable connector. The other port in the graphics card, an HDMI port, can be connected to an LG42LK450 LED TV. using a twenty-five foot long HDMI cable. The TV requires 1920 x 1050 resolution to show true High Definition. Without the HDMI cable the grapics card supplies 1920 x 1050 resolution to the TV and 1680 x 1050 resolution to the PC monitor. Windows shows High Definiton (when available) and Ubuntu > System> Monitor shows the Sasmsung monitor has 1680 x 1050 resolution. Everything works as it should work. With the HDMI cable connected when I boot into the Windows 7 partition, and select HDMI input on the TV, the TV and the computer monitor show what would normally be on the computer screen, be it the desktop, a website, whatever. If I leave the HDMI cable connected and try to boot into the Ubuntu 10.04 LTS partition, the ubuntu screen appears briefly then goes blank. The TV then says "No Signal" until I select the source as TV, not HDMI. I believe that for reasons I do not understand, Ubuntu 10.04 LTS does not provide a high enough resolution when the HDMI cable is connected, causing the screen to go blank.

Finally, my question is, "How can I get to that Ubuntu screen and raise the screen resolution with the HDMI cable connected?