Ok, so you've done the math and figured out that your head is really worth the protection given by a bicycle helmet.  Good news for you there has never been a better time.  Gone are those big ugly white and orange eggshells that go well with running shorts and knee high tube socks...so 70's and 80's.  Today the helmets are lighter, contain more vents, no longer use that ugly tape to keep the plastic on, and come in a variety of sizes and colors. 

First of all a helmet that has not been involved in an accident can last a minimum of 5 years if kept in good condition.  Bicycle helmets are designed for one accident only unlike skate type helmets.  That beer cooler looking styrofoam is exactly that and is designed to absorb the shock and break apart to protect your head.

Most helmets are made of EPS  (expanded polystyrene) foam covered with a thin plastic shell. Older helmets had this shell taped on but now on most new helmets the shell is applied directly to the EPS core.  Without getting to graphic the plastic shell does two things; it keeps the EPS foam from disintegrating into a million different pieces after the initial impact, and it provides a way for your head to slide easily across the street or pavement so that nothing snags. 

Just like shoes different manufactures use different molds to fit differently shaped heads.  Try on more than one brand to see which fits you the best.  Make sure that it is snug but not too tight and that you wear it level on your head.  Most helmet manufacturers make three types of helmets for three types of users; recreational, mountain, and road.

Recreationall Helmets:  These are the least expensive out of all of the helmets but are excellent for someone who doesn't ride a lot, is on a budget, or is completely satisfied with the fit.  These helmets usually cost no more than $60 and are sized as "One Size Fits All".  Vents are usually limited but since most recreational riders to not ride for very long or with little intensity this is perfect.  Also there is no emphasis on weight due to the limited time spent on the bike.  Due to the slow speeds and upright riding position these helmets almost always come with a visor.

Mountain Bike (MTB) Helmets:  These are similar to recreational helmets in one way; the use of a visor due to the more upright riding position and slower speeds.  The sizing however changes form a One Size type to Small, Medium, and Large.  The are usually more agressive, larger vents due to the duration on the bike and the fact that MTB riders may ride for up to an hour on very steep hills at slow speeds, neccesitating the larger vents and a way to manager the riders sweat (also known in the business as sweat mangement).  MTB helmets also tend to protect the back of the head due to a greater chance of this type of fall than say a road rider.

Road Helmets:  These helmets strees aerodynamic qualities and lighter weight than the other two categories.  Since road cycling is generally faster than mountain or recreational riding aerodynamics is key.  Pick up a MTB helmet sometimes and then a road helmet.  The MTB helmet is usually larger with large vents in contrats to the road helmet with  no visor, its smaller profile and smaller, narrow vents.  Road helmets like everything else on the road bike is lighter.  The Atmos by Giro which Lance Armstrong has worn in the Tour is only 240 grams whereas recreational helmets can top 300 grams.  Due to speeds sometimes exceeding 30 miles per hour larger vents just aren't neccesary.

Like most everthing I recommend with gear, please try to emphasize safety and fit (otherwise known as function) over color or graphics (otherwise known as fashion).  Beleive me, you will not care what graphics are on the helmet when your head is hot and sweaty on a long climb and your helmet begins to hurt because it has never fit you very well.  The last thing you want to do is take it off due to discomfort.  Hope this helps.