How to connect LG LK450 TV to Ubuntu 10.04 PC?

I have a PC which can boot to Windows 7or Ubuntu 10.04 LTS. I have an AMD XFX graphics card graphics card in the PC which has two port. One port is connected to the computer monitor, using the computer monitor's cable. The other port, an HDMI port can be connected to the LG42LK450 LED TV using an HDMI cable. When the HDMI cable is connected between the computer and the TV and I boot to Windows 7, both the computer monitor and the TV show what would normally be on the computer screen, be it the computer desktop, a website, whatever. But when I boot to Ubuntu 10.04, the computer monitor shows the Ubuntu opening screen briefly, then goes blank. The TV screen says "no signal." The only way I can get a screen on Ubuntu and work in ubuntu is to disconnect the HDMI cable from the computer, and reboot.
I believe the problem is Ubuntu provides only 640 X 480 resolution where the Samsung uses 1680 X 1080 resolution. (The TV ideally uses 1920 X 1080). I believe if I could get the Ubuntu 10.04 to use 1680 X 1080 resolution everything would work perfectly.
The Samsung would probably accept a lower resolution, but not show as good a picture.
If you could show me how to have Ubuntu 10.04 show a higher resolution, my problem would be solved.