NEW YORK (MONEY magazine) - More than any CD changer, more than any heated or ventilated seat, the most sought-after option in a car these days is a full complement of safety features.

Here are the ones that are definitely worth the cost, plus as a few that are good but need to cost less before they'll be really useful.

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Whether they're standard features or on the options list don't buy a new car unless it has all of these features:

Side and side-curtain air bags

Modern cars do a great job of protecting you in frontal collisions, but that's relatively easy -- there's a whole engine compartment between you and the other car. Side impacts are the tricky ones. With only a thin door between you and an oncoming vehicle, things can be much worse.

That's why side air bags (which protect the upper body) and side-curtain air bags (which cover the window area and shield your head) can make a critical difference in a collision. The good news is that side and curtain bags are becoming standard fare on even some budget cars, such as the $11,000 to $14,000 Kia Rio.

The bad news is that they remain optional in far too many models. And a distressingly low percentage of consumers choose to pay extra for additional air bags, though they typically add a meager $250 or $300 to the cost of a car -- not much more than a set of floor mats.

Check off the option box for air bags, every time.

Electronic Stability Control (ESC)

ESC technology uses existing anti-lock brakes plus motion sensors and software to recognize a skid before it even begins. Then it applies individual brakes and adjusts the throttle to restore control. Make no mistake about it: ESC works. Countless times, I've hit a slick patch or taken a racetrack turn a bit too fast and felt ESC instantly pop the car back on course, usually before I even knew something was amiss.

If you're shopping for an SUV, an ESC system is even more beneficial, since those vehicles have trickier emergency-handling characteristics and can have more of a tendency to roll over.

Comparing models before and after ESC was installed, the National Highway Traffic Safety Administration found that the technology reduced crashes by 35 percent in cars and by a stunning 67 percent in SUVs. As ESC has become widely adopted, prices have fallen. Where they're still an option, most ESC systems now cost you $500 or less.

It's the best money you'll ever spend on your car.

Winter tires

The results are clear. Winter tires are the best way to enhance driving safety on snow and ice. Period.

Recently, online tire retailer the TireRack ran its own test with two Volkswagen Touaregs. One was shod with its factory all-season tires, one with Bridgestone winter tires. When both came to a panic stop on ice from just 35 mph, the VW with all-season tires took 70 feet longer to come to a halt -- about the length of a school bus.

Don't be misled by all-season tires.

"All-seasons are designed to be good at everything, but they aren't great at anything, especially winter driving," says Matt Edmonds, V.P. of the Tire Rack. Tires that meet the winter standard carry a symbol of a mountain peak with a snowflake and cost around $100 a tire.

To keep things simple, you'll want to buy an inexpensive set of wheels (figure $60 to $100 per wheel) to mount the winter tires instead of switching tires back and forth on your standard wheels. And always use winter tires in sets of four -- using a single pair on the drive wheels can create unbalanced handling.

These intriguing, useful technologies are, so far, available only on higher-priced cars.

Rear-view monitor

My initial reaction to a monitor in the dashboard that could show a view of the space behind your car was that it was technology for technology's sake.

On subsequent tests, however, I began thinking about families with children -- and pets, and Big Wheels and anything you might overlook while backing up, possibly with tragic results. After several auditions, count me a believer: Rear-view monitors go a long way to boost safety and confidence, especially for drivers who feel anxious backing up a jumbo minivan or SUV (most of us).

The best system, from Nissan and Infiniti, superimposes a green-yellow-red grid over the screen view (red being the zone closest to your rear bumper), making it easy to judge distance and snuggle to within inches of the car or wall behind you. Not only does it keep you aware of what's behind you, but it'll keep you from ever scratching another bumper during a parking maneuver.

The caveat is cost: Every monitor currently available is bundled into an option package that forces you to buy a pricey navigation system and usually some other gizmos, at a cost of several thousand dollars. So if you really want this safety feature, you're forced to pay for other things you may not want.

Pre-collision systems

Even before that first crunch of a fender-bender, the latest luxury models are trying to protect the people inside. Mercedes, which pioneered pre-collision technology in 2003, has enhanced it on the new 2007 S-Class.

If radar sensors detect an impending collision, the S-Class will, in less than a second, shut the windows to support the curtain air bags (unless the window is obstructed), reposition seats for optimal crash performance, and retract seat belts to cinch occupants into place. Right now pre-collision systems are limited to luxury models, but as the technology becomes more common, expect mid-price models to begin offering similar systems before the end of the decade.

Active braking

Taking pre-collision systems a step further, the 2007 Mercedes S-Class and the 2006 Acura RL use the car's radar "vision" to brake the car when it senses that you're not doing the job. That may sound scary, but the systems actually work safely and unobtrusively.

The technology is not perfect yet (it often doesn't recognize a completely stationary car ahead), but my testing convinced me that it could make the difference between a minor fender bender and an injury-producing accident.

As a $3,800 option on the '06 Acura RL, the car's Collision Mitigation Braking System first warned me that I was heading too quickly toward upcoming cars by flashing a huge brake-warning on the driver's display and sounding a chime.

If I still didn't stop, the Acura tugged back on my seat belt, then braked forcefully but smoothly, still giving me ample time for a safe stop on my own. (The system won't brake the car to a full halt and can be switched off from the dash.)

The new Mercedes S-Class has a similar system, but it springs into action only after you've initiated braking. The car then uses its onboard radar sensors to calculate the optimal brake force required to avoid a collision, and applies that amount if you're rapidly closing in on another object.

The same system also works in conjunction with the car's radar-based cruise control. In that case, the car will maintain a set distance from the car in front of it, adjusting its speed from 0 to 125 mph.

When I used it, the Benz accelerated and braked for hours without requiring me to touch either pedal, even in heavy stop-and-go traffic. Neat stuff, but these systems will become truly effective only when they're cheap enough that most cars have them. Right now you may be able to avoid a rear-ender, but it doesn't do you much good if no one else can.

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