Choose a design based on where you would like to wear your headphones. Ear buds fit inside your ear, while over the ear headphones wrap around the outside of your ears. Behind the head headphones fit into your ears and rest below the back of your head. Over the head headphones are standard.
There are two kinds of networks types: CDMA and GSM
CDMA: CDMA is predominantly limited to the US and parts of Asia. It is not supported throughout the European Union because of regulatory restrictions.
GSM: GSM is the a much more international standard and works across most geographies including the european union.
If you are looking to travel mostly inside US you should stick with CDMA because it offers a much better coverage than GSM, especially in the rural areas. However, if you plan to travel internationally, GSM might be a better option because of much broader support.
Each network type is further supported over different frequencies - also called as bands. The different bands include 800 MHz, 850 MHz, 900 MHz, 1800 MHz and 1900 MHz. Note that 800 MHz and 850 MHz are actually the same band and are used interchangeably. GSM Quad-band cell phones support the 850,900,1800 and 1900 bands and are best suited for international roaming.
Installed memory is the amount of available memory that comes with the PDA. While you can usually add more memory through an expansion slot, you want a PDA that has enough memory to run the programs you intend to use and to store your personal information.
Internal antennas are built into the device and can not be changed or directed. External antennas can be directed to point in different directions; this may be used to increase range of the device. Detachable antennas are screwed into the device and can be upgraded or replaced easily.
Most PDAs have touch screens that require a stylus to enter data. You can also find devices with a Jog Dial, that allows you to scroll through pages and make selection quickly. Still other PDAs have full keyboards.