Greenwood Heating and Air and Brennan are good contractors, but you may want to go through PSE if you already have gas to you home, they have pretty good deals for new units. If you use a local HVAC company, make sure you get a brand new unit, not one that has sat around for awhile in a warehouse. It is not good for HVAC units to sit and it voids the manufacturer warranty if not installed right away.
A friend of mine recently got a bid to replace a unit that was installed in 2006. Turns out, the unit that was installed in 2006 was manufactured in 1998.
Be careful and remember, you get what you pay for.