What TV do you have? It is very possible that your TV does not accept Computer resolutions on the HDMI ports. And/or see if one of your HDMI ports is says "DVI" on it. One of my Panasonic Plasma's does not like some common "computer resolutions" unless I use the one HDMI port that says "DVI" on it.
Of course there is always the option of using a VGA connection (unless you want the ease of audio through HDMI).