I would start out first by asking what you plan to use the GPS for most often.
Like JB indicated, you'll need a GPSr that bluetooths over to your phone. They are often referred to as "Pucks" since there are no displays on the devices. They simply transmit and receive data from the GPS system and send it to your phone via bluetooth.
In order to make use of the info in any meaningful way, you generally need a data plan. The GPS info is translated onto Google Maps, TeleNav, or some other map utility (that pulls the map info down in realtime over the data connection) that lets you know where you're going.
If you simply want GPS to help with driving directions, a dedicated unit like a Garmin Navi or Tom Tom might be better for your needs.
If you live in an area where Coverage is spotty (rural areas, etc) the Puck won't do you much good for driving, since the map programs would stop working once you lost your wireless signal.
If you still think a GPSr Puck would be a good fit for you, prices on eBay can be excellent. Stick to the common sense guidelines, (quality sellers with PayPal, etc) and you can get a decent unit. I've heard very good reviews from Stackers using the Holux devices, but I do not have any personal experience with them.
If I knew where I was going, I might already be there. -- Cross Canadian Ragweed.