So you want to buy a smart phone? After enduring the taunting of your friends, your contract is finally up on your creaky 3-year-old not-so-smart phone. The device doesn’t do e-mail or the Web, and it can’t find your position on a map.

There is only one problem. Now that you have decided to step into 2007, the choices baffle you. The mobile phone industry has changed enormously since you last shopped. There are several new companies making devices, the phones have an alphabet soup of hardware options, and they run software that can do all kinds of amazing things.

For some shoppers, though, the powers of modern smart phones may be overkill. Perhaps you would like to occasionally send e-mail or get access to the Web from your phone, but you don’t really care about playing games or editing movies. For you, there is an entry-level smart phone, a phone that will let you do some of the fantastic things your pals keep bragging about, but that won’t cost too much or require a college course in using it.

Cheaper than $100

For the last few weeks, I have been testing several phones from the four major carriers — AT&T, Verizon, Sprint and T-Mobile. Although there is no official definition for what makes a phone “smart,” the industry does have a few generally recognized criteria. A smart phone is usually capable of getting access to your e-mail. It can display Web pages in a way that doesn’t distort them too much, and it can connect to the Internet using a cellular data connection (known as 3G) or your home wireless system (Wi-Fi).

I defined “entry level” as any smart phone that costs less than $100 when you sign up for a new contract. These parameters did, of course, exclude the kings of the smart phone market — Apple’s iPhone 4, Droid 2 from Motorola and the HTC EVO 4G. But you will be surprised by what you can get for not much money upfront. A few of the best phones that I looked at cost less than $50, and some were completely free with a new contract.

Data plans

But that is just the price of the phone. When you step into the world of smart phones, the real sticker shock comes on your monthly bill. Most carriers require you to sign up for a data plan when you buy a phone that provides access to the Internet. These can add $15 to $30 a month over what you are used to paying for your dumb phone.

Of the four carriers, AT&T’s smart phone plan is the most affordable. For $54.99 a month, AT&T will give you 450 minutes of voice calls and 200 megabytes of data usage a month. (Some observers have grumbled at AT&T’s limit on data usage, but for entry-level users, 200 megabytes should be more than enough.)

By comparison, Verizon’s cheapest plan, which includes 450 minutes of voice calls and unlimited data, costs $69.98 a month. Sprint’s unlimited data, 450-minute plan is $69.99. T-Mobile’s lowest-priced data plan — 500 minutes of voice, unlimited data — is $79.99 with a two-year contract.

Now let’s talk about the phones themselves. The first major decision you have to make concerns the keyboard. Do you want a touch-screen keypad, as on the iPhone, or do you want a physical keyboard with push-button keys? There are trade-offs on both sides.

An onscreen keyboard lets you have a bigger screen on a thinner, sleeker device, while a hard keyboard gives you tactile feedback as you type. Although there are strong feelings on the question, your best bet is to try both keypad styles at the store. The one you prefer will usually be a matter of taste. It most likely will depend on your own sense of coordination and the size of your fingers.

Last year’s iPhone

If you want a touch keyboard, the best device you can buy for less than $100 is the iPhone 3GS. That is last year’s iPhone model, which Apple sells for $99 with a two-year contract, and which offers nearly every feature available in the new iPhone 4. It is also one of the simplest phones on the market to learn to use. And if you do decide to venture into the wide world of add-on apps, there is no better phone.

If $99 is too steep, I suggest the LG Ally, from Verizon, or the AT&T Motorola Backflip, each of which sells for $49.99 with a contract. These phones run on Google’s Android operating system, but you don’t need to know what that means in order to use them. Though neither is a looker — they are a bit bulky in your pocket — and they are noticeably slower than more expensive phones, I found each quite handy for Web surfing and e-mail. They both have well-designed keyboards.

Another low-price phone to consider is Palm’s Pre, which was released last year to great acclaim as a potential “iPhone killer.” The Pre did nothing of the sort, but it remains a lovely phone to use — and now you can get an updated version, the Pre Plus. It is just $29.99 from Verizon.

It is unclear how long Palm will continue to make the Pre Plus (the company is undergoing a merger with Hewlett-Packard) so if you admired the Palm last year, now is a good time to snap it up. (The Pre’s underpowered little brother, the Palm Pixi Plus, sells for even less. Both Verizon and AT&T offer it free with a contract.)

Other choices

There are also some options on the nation’s two smaller carriers, Sprint and T-Mobile. T-Mobile has introduced the Motorola Charm, a square Android phone that will sell for $74.99 with a contract. I found the phone’s unusual shape made for more difficult typing but, other than that, it was a solid phone. So, too, was the Samsung Intercept, a $99.99 Android phone offered by Sprint.

If you have heard about Apple’s troubles with its new iPhone, you might have one question about inexpensive smart phones. Forget the bells and whistles, how well do they work as phones? Unfortunately, that is not a question I can shed much light on. Your call quality will depend greatly on your geography and the specific location of cell towers in the places you frequent most. In other words, it is hard to know how well your phone will make calls before you actually try it.

But remember that all major carriers offer a 30-day grace period on your contract. If you notice your phone acting up in that period, take it back. At the least, a smart phone should be smart enough to let you make calls.