If your existing alarms are over 10 years old, it's a good idea to replace them.
There are two types of smoke alarms, ionization and photoelectric. Ionization is the more common type and uses a radioactive material to detect smoke. This type is more effective at sensing fast, flaming fires. Most ionization detectors can be purchased for about $10 to $15
Photoelectric detectors use an infrared light source in a chamber with a photocell. Smoke entering the chamber would reflect the light onto the photocell which would trigger the alarm. Photoelectric types are more effective for slow, smoldering fires. They are also slightly more expensive at about $25
There are also "dual sensor" detectors that incorporate both types of sensors so you have the best protection for both types of fires.
The newest trend are "wireless interconnection" such as the First Alert "one link" series which use radio communication between units so when one sounds, all of them go off. Originally, interconnection was only available with 120 volt hard wired units. A normal smoke alarm that is in your bedroom may probably not do you any good if a fire were to start in a separate part of the house as it will take time before the smoke reaches your room. With an interconnected system, you could have an alarm closer to where the fire originates go off which would also signal the alarm in or near your bedroom.
At the bare minimum, I would purchase the two pack and install one in the bedroom which you would normally occupy and another in the living room or close to (but not directly in) the kitchen where most fires usually occur. The "best" would be to install one in every bedroom, one in the hallway outside the bedrooms, one or two in the living room (depending on size) and one near the kitchen.
http://www.firstalertstore.com/store/start.asp?area=nav_acc_model&category_id=59
and last night next door neighbor set his place on fire. 
:drunk:
