Your notebook's add-on slot will dictate the right type of 3G card for you. Here's a look at the available options.
PC Card: Most notebooks made in the last decade feature a PCMCIA Type II, or PC Card, slot. About the width of a credit card, a PC Card slides into the side of your notebook and draws power from your notebook's battery.
ExpressCard/34: Smaller than a PC Card, an ExpressCard/34 device also slips into the side of your notebook. ExpressCard is common on newer notebooks only.
USB: These days Universal Serial Bus is, as it name says, universal on PCs and Macs. While many ultralight notebooks may not offer either a PC Card or ExpressCard slot, you'd be hard-pressed to find any modern computer lacking a USB port. USB data adapters are a great choice if you're planning to use a single device with a variety of different computers.
Built-in: As 3G Internet connectivity has grown, so have the options for delivering it to your PC. Many major manufacturers, including Dell, HP, Lenovo, Panasonic, Sony, and Toshiba, now sell laptops with built-in mobile broadband adapters for EvDO and UMTS/HSDPA. Built-in mobile broadband typically adds about $180 to a notebook's retail price.