(MONEY Magazine) – By now, you're pretty savvy when it comes to buying a new car. You know all about dealer-invoice pricing, depreciation and extended warranties. There's still one last chance, though, for the dealer to get the better of you. A new model's options list can be a minefield, and if you're not careful you could end up spending 20% or more above what you intended for your car. So it pays to know whether an option is good for you or just for the dealer.

The most popular options fall into three categories: must-haves, ones you might want depending on your needs and ones better purchased on the aftermarket, where you'll find just as good (if not better) features for a fraction of the price. Haggling over the sticker is only part of the game—with this guide, you can make sure you're getting a car that's well equipped and priced just right.

REARVIEW CAMERA › TYPICALLY SOLD AS PART OF AN EXPENSIVE OPTIONS PACKAGE WORTH IT? MAYBE

SIDE AIR BAGS › TYPICALLY $600 WORTH IT? YES

RUN-FLAT TIRES › TYPICALLY $400 WORTH IT? MAYBE

REAR-SEAT DVD SYSTEM › TYPICALLY $1,300 WORTH IT? NO

BLUETOOTH HANDS-FREE PHONE KIT › TYPICALLY $300 WORTH IT? MAYBE

GPS NAVIGATION › TYPICALLY $2,100 WORTH IT? NO

ELECTRONIC STABILITY CONTROL › TYPICALLY $500 WORTH IT? YES

THE MUST-HAVES

Electronic Stability Control

› This safety feature is so good it's already standard on half the cars sold in the U.S. today. By applying the brakes to individual wheels when sensors detect a loss of control, ESC helps keep your car from spinning into a guardrail (or worse). On models where it's offered as an option, such as the Mazda 3 and Cadillac CTS, it'll run you an extra $500 or so. ESC is quickly joining seat belts and ABS as part of the safety triumvirate that every car should have. In five years you won't have a choice: ESC has been slated to be mandatory by 2012.

The Verdict The best $500 you can spend. If a car you're considering doesn't offer ESC, then reconsider that car.

Side Air Bags

› Front air bags get all the cool crash-test footage, but side air bags are no less important for occupant safety. There are two basic types of side air bags: ones that protect the head (side-curtain air bags) and ones that protect the chest (side-torso air bags). A car can have curtain air bags, torso air bags or a combination of the two. According to the Insurance Institute for Highway Safety, side-curtain bags have reduced driver deaths in side collisions 37%; side-torso bags have reduced driver fatalities 26%. Since 1999 a voluntary standard has minimized any risk side air bags posed to children; go to safercar.gov to see if your model meets that standard. Side air bags will add around $600 to your negotiated price but, really, is this an area of the car where you want to be cheap?

The Verdict The more your car looks like it was attacked by the Stay Puft marshmallow man in the event of a crash, the better off you'll be. Spend the money.

THE MAYBES

Run-Flat Tires

› Nobody loves changing a flat tire, but some hate it more than others. If you fall into the latter camp, run-flats may be a good buy. Offered on cars from the Mini Cooper to the Rolls-Royce Phantom, run-flat tires let you drive up to 100 miles at 50 mph even if a tire has lost all its air. There's no spare in the trunk (which can give you more space for your gear) and you'll never have to crouch on the side of a highway at 11:30 p.m. That said, run-flats can cost as much as 50% more than regular tires (it's a roughly $400 option on most cars), not every tire shop is certified to handle maintenance and getting a run-flat patched after a blowout can cost upwards of $50—twice the price of a standard patch job.

The Verdict It comes down to how often you expect you'll change a tire and whether you'd pay around $400 to avoid it.

Bluetooth Phone Kit

› In many states it's now against the law to talk on a handheld phone while driving (cruising while eating a Big Mac, however, is totally legal everywhere). The Bluetooth option, available in cars such as the Chrysler 300C and Nissan Versa, means you can use a wireless connection and steering-wheel-mounted buttons to place and receive calls. On factory-installed Bluetooth systems such as Audi's, your phone-book info is also downloaded to your car's in-dash display so you can scroll through a call list. Most systems incorporate voice-recognition technology, so initiating a call is simply a matter of saying "Call home." Of course many cell phones already have built-in (albeit tinny) speakerphones, and some offer voice recognition as well. And you can buy an earpiece for next to nothing. Factory-installed Bluetooth systems can run around $300, while a regular phone with similar features may cost $50—that's a pretty big spread.

The Verdict If you're conducting a lot of business from the driver's seat and need a crystal-clear, full-featured hookup, your car's Bluetooth is the answer. If you're just calling the family to say you're heading home, a handset's speakerphone or an earpiece should do the trick.

Rearview Camera

› Rearview cameras are immensely helpful—you get a wide-angle view of everything behind your rear bumper, whether it's another car or a tyke's Big Wheel. So why isn't it a must-have? The problem is that rearview cameras are often packaged with other options that aren't nearly as useful. On the Infiniti M35, for example, a backup cam is part of the $2,950 "Technology Package," which includes in-dash navigation and an upgraded stereo. It's one thing to package some sybaritic luxuries together (if you want massaging seats and a backseat refrigerator, that's your business), but automakers shouldn't play cute with something that actually contributes to pedestrian safety.

The Verdict It's a genuine advance, but just try finding it à la carte. Unless you really like the other options included in the package, spend a little extra time looking behind you when you back up.

THE AFTERMARKET BUYS

GPS Navigation

› GPS lets you find your way almost anywhere, and that's great when you're headed somewhere unfamiliar, but do car manufacturers really have to gouge you so much? There are good aftermarket navigators that do everything factory-installed models do, if not more, and they cost a fraction of the price. Want navigation on your BMW 3 Series? Prepare to cough up an extra $2,100. Meanwhile you can get Magellan's Maestro 4050 portable navigation unit for $700. The Maestro has voice recognition and live traffic updates and acts as a Bluetooth speakerphone for your mobile. It requires no installation (save a suction cup or sticker), so you can swap it between cars or lend it to friends and family. And the Maestro 4050 is a top-of-the-line model—you can get a perfectly decent unit (such as the TomTom One) for around $300.

The Verdict Buy aftermarket. Blow the difference on a great road trip, knowing you won't get lost.

Rear-Seat DVD Systems

› Let's say you're in the market for a Honda Odyssey minivan. It's an excellent choice for families, so long as you steer clear of the DVD option. Honda charges $1,300 for a ceiling-mounted movie player, a fold-down nine-inch screen and a pair of wireless headphones. All for the luxury of having the screen fold back up into the headliner once the brood is done watching Shrek 5. A nine-inch portable DVD player bought at Best Buy costs about $180. Throw in two pairs of headphones and the price creeps up to $200. And like the portable GPS unit, you can take this handheld movie theater with you wherever you go.

The Verdict Save the $1,000 and go for the portable. Then you can buy all the DVDs the kids want.

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