It depends on the screen size you go for.
Ratios are just that, ratios. Taking a typical TV for example if you literally take a tape measure and measure the height of a standard aspect screen you would need a much larger widescreen to get the same height because the screen is measured on the diagonal.
Putting it simply if you want the same height you will need to invest in a big screen. given that many web sites are designed for standard format screen (look at the BBC site as an example) on widescreens you will get more "dead" space at each side.
More site will go widescreen eventually but the real advantage now is for watching DVDs which have a widescreen format of for Vista which benefits from wiedescreen with more space for the sidebar with gadgets.