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Platform: Three different types of PDAs exist now: the organizer with some PC features (Palm); the handheld PC with organizer features (Pocket PC); and the wireless e-mail/pager with organizer features (RIM BlackBerry). Though these platforms used to be distinct, the boundaries have blurred. Some formerly BlackBerry-exclusive functions, such as persistent connections, have found their way into Palms (the Palm I705), and numerous Palm and Pocket PC-type apps have crept into BlackBerry systems.

Operating system: How you plan to use a PDA can suggest the operating system you should choose. Palm OS--based devices--such as PDAs from Palm, HandEra, Handspring, and Sony--dominate the PDA market because of the operating system's simplicity. Unlike Windows or the Pocket PC OS, the Palm OS cannot multitask: It opens just one app at a time. Unless you use a third-party synchronization utility, the Palm OS will synchronize appointments and contacts only with its proprietary PC PIM, the Palm Desktop; most Palm OS vendors bundle an Outlook filter to sync with that desktop PIM. You'll need Dataviz's Documents to Go, Cutting Edge Software's Quickoffice, or similar software to create and edit Microsoft Office-compatible documents on a Palm OS device.

Microsoft's Pocket PC is basically a considerably shrunken version of its desktop Windows OS and includes Pocket PC versions of Microsoft Office applications such as Outlook, Word, Excel, and PowerPoint that let you create and edit documents as you would on your desktop. The integrated Windows Media Player handles MP3, WMA, and other multimedia files. Pocket PCs are available from Casio, Fujitsu, Hewlett-Packard, and Toshiba, among other vendors.

The RIM BlackBerry OS is primarily a wireless e-mail system that synchronizes e-mail messages and calendar items with the major enterprise-collaboration environments, Microsoft Exchange (Outlook) and Lotus Domino (Notes).

Internal memory (RAM): Basic organizer functions (appointments, alarms, contact lists, tasks, and memos) use a fraction of the 8MB of RAM that most Palm OS PDAs hold. Color applications require more memory, however, and adding third-party utilities and games quickly eats up RAM, so look for a 16MB Palm OS device if you plan to load lots of extra programs. Apps on Pocket PC devices require even more memory, especially to play audio or video; consider a unit with more than the minimum 32MB of RAM, preferably 64MB. You will still need an external memory card as well, to load your digital music and images on.

Expandability: Most PDAs other than BlackBerry devices carry external expansion slots to accommodate additional storage in the form of CompactFlash cards, Secure Digital cards, or Memory Sticks. (Early Handspring models included a proprietary Springboard module slot.) These slots can also hold add-ons such as cameras, MP3 players, phones, and GPS mapping antennas.

Input (keyboard or stylus): The handwriting recognition capabilities on Palms and Pocket PCs have improved over the years, and all PDAs come with a virtual hunt-and-peck on-screen keyboard. Still, many users have turned to one of the numerous third-party add-on keyboards. Some enterprising vendors such as Sony and Handspring have put thumb keyboards on their latest PDAs--without eliminating styluses--so you can choose the method of input you prefer.

Size and weight: Ideally, a PDA would fit in a shirt pocket without stretching it out of shape. But while a few can do that, many PDAs are too bulky and heavy to be called truly pocket-size. The lightest weigh about 4 ounces and measure about 4 inches high, 3 inches across, and 0.5 inch thick. A dress-shirt pocket can handle that easily--give or take an ounce. Any of the Palm-brand models, such as the M515 or I705, fit the bill (and the shirt) well. The Toshiba Pocket PC E310 does too. Though Sony's Clié PEG-NR70V is thin, at 7 ounces it is a tad too heavy for a pocket. The bulky 0.75-inch-thick Compaq IPaq stands at the other extreme.

Battery power: Bright color screens definitely make the PDA experience more pleasant, and they support more software choices than monochrome screens do. While Pocket PCs once laid claim to the highest-resolution TFT screens, some Palm OS PDAs now do as well. Currently two different technologies provide bright displays. The more common is backlighting, which illuminates the screen at some cost to battery life. Reflective screens, the other technology, still have backlighting but work well in outdoor or brighter ambient light without it. Before purchasing a PDA, check to see how well you can read the screen with the backlight off.

Rechargeable batteries, typically lithium ion, are more efficient today, as are the power control options on PDAs, but most monochrome and lower-end models still use AAA batteries. That said, you can expect up to a month of power between charges from some monochrome units, but no more than 10 hours of life from color-screen units with typical usage.

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