USAirways isn't building a terminal at LGA. The Port Authority is rebuilding the Central Terminal at LGA and will start construction in 2014.

However USAirways if they merge with AA will be found at the American Airlines terminal.

Which means Delta will have more gates available at its own terminal. So either Delta uses all the gates at both their terminals at LGA or has another partner come in to fill the rest of the terminal.

American Airlines/USAirways if they merge will be found at Central Terminal at LGA Airport which will be under construction.