Consumer electronics is such a rapidly evolving, constantly changing marketplace that it's always wise to consider whether to commit yourself to a built-in system that will be with a vehicle even after it's obsolete.

Beyond that, if your $2,495 entertainment system's price is folded into a typical car loan, it's going to cost you even more. Plug that number into MSN Autos' Loan Calculator, and, assuming your loan's interest rate is running at 6% for a 60-month term, you'll find the entertainment system will add $48.24 to your monthly payment and wind up costing you a total of $2,894. That's nearly $400 in interest alone.

Factory GPS systems are notoriously expensive: To get a factory system in a Ford Mustang means opting for a $2,340 electronics package. Meanwhile, a portable GPS system from Garmin or TomTom runs a couple of hundred bucks and can be moved from car to car. Or, as is becoming more common, you could just use the GPS system in your cellphone.

Enduring through the ages

"The most important options are those based on safety and convenience," used-car dealer Lang says. "You should focus on the options that endure through the ages -- things like power windows, power door locks, anti-lock brakes and traction control."

In fact, those options have grown so popular that today it's tough to find many new cars without them. Electronic stability control is mandatory on all new cars sold in the United States, starting with 2012 models. And since anti-lock brakes are required standard equipment on all cars sold in European Union countries, virtually all vehicles are sold with them in North America as well. Only the cheapest economy cars today are still offered with manual crank windows.

"At the low end of the used-car market," Lang continues, "the presence of ABS (anti-lock braking systems) and traction control are great selling points. And everyone wants power windows and power door locks."

Leather upholstery might be nice to have now, but it's not likely to impress value-conscious used-car buyers when the car is a couple of years old. And it can be deceptively expensive. To get leather in a 2011 Nissan Altima S, for instance, means not only replacing the upholstery as part of the $1,540 SL package (which includes heated front seats, mood lighting and other extras) but also accepting a mandatory $1,350 "convenience package" that includes an eight-way power driver's seat, steering wheel audio controls, automatic on-off headlights and a Bluetooth hands-free phone system.

Plus, that Altima will also need the $1,150 "convenience plus" package that adds a power sliding moon roof, dual-zone automatic temperature controls and folding outside mirrors with LED signals. And, oh, yeah, you'll have to get the $140 splash guards, too.

So to get leather upholstery in a $23,330 Altima S means buying a $27,510 Altima SL.

Buying dealer-installed trouble

Once you've avoided the pitfalls of the factory-option packages, get ready for the challenge presented by dealer-installed options and accessories. These options are big profit centers for dealers, and many of them are notoriously sketchy in value.

Because manufacturers warrant their vehicles against corrosion, they are obsessed with rust-proofing. GM, for example, covers its vehicles for six years or 100,000 miles against rust-through. There's no treatment a dealer can apply that will add much to what the factory has already built in.

Fabric and material protection can be bought in the form of a bottle of Scotchgard -- and usually for hundreds of dollars less than what a dealer charges. Extended warranties usually aren't worth the money unless you plan on keeping a car for decades. And even then, such warranties are available for hundreds less through sources outside a dealership.

Accessories such as alloy wheels, floor mats, deck lid spoilers and various dress-up items are also available for hundreds or thousands less outside the dealer. For instance, a set of 18-inch alloy wheels with tires offered through Honda dealers for the Accord carries a manufacturer's suggested retail price of $2,325. Similar wheel and tire packages from online outlets like Tire Rack go for hundreds less, offer vastly more wheel design choices and can be shipped ready to install to a local tire shop.

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As a general rule, it's best to say no to every dealer-installed option and accessory. There are virtually always less-expensive outside options. And you can always go back to the dealer later and purchase products through the parts department if you change your mind.

The power of low expectations

Buyers often go into dealer showrooms aiming to buy the most lavishly equipped car they can. They then work their way down to what they can afford -- and spend every last dime they can. They feel the loss of every option they can't afford.

Instead, a better strategy may be to start with the barest car possible and consider each option as an expensive addition that you may not need. Then you can add some things to your car as time and money permit. In the meantime, you'll keep the purchase price (and monthly payments) for your new car down.