What Do I Need? --in order to get GPS navigation working..
Make sure you have the following:
GPS Antenna, GPS Receiver, GPS Software, GPS Maps
Most GPS gizmos today combine the antenna and reiver
Most GPS software comes with maps (just make sure your area is covered by the maps provided)
Make sure the GPS Unit can connect to your computer (serial, USB, Bluetooth, etc) and that there are drivers (if required).


Some Pitfalls to avoid:
Buying wrong hardware for your computer :: Buying Serial GPS for a PocketPC that doesn't have a serial port
Buying (aquiring) software that only works w/ a specific GPS Antenna/receiver set
WindowTinting :: Some (metalic) window tinting may interfere with the GPS signal receiption. Plan your antenna placement carefuly to avoid this.


Features to consider when buying GPS Navigation software:
Maps :: Originaly, they come from one of two sources. This means that copilot, delorme, tomtom, etc do not make their maps. THEY DO control the level of detail interpreted by their maps, they DO control how the data is displayed. So, you may find some more to your liking than others.
Display :: Most directly impacts the usability of your NavSystem. If the features and tags are clear and simple to read. If the directions are clear to see and hear (if voice enabled).
Touch Screen Friendly :: Not all GPS Software was written with the TouchScreen user in mind. Take care to find one that suits your needs.
Update Speed :: While driving it's generaly usefull to have your navigation system keep up with you. If your GPS software can't update at least once per second then you risk missing turns while your NavSystem is catching up.
POIs :: Points Of Intrest help you find resturants, gas, banks/ATMs, etc.. a 'healthy' POI database can really brighten your NavSystem.


Features to consider when buying GPS Hardware:
USB/Bluetooth/Serial :: Most GPS devices connect to the computer via USB. (Some are Bluetooth, others are serial, CF, PCCard, etc) Make sure you get the one that suits you best.
DeadReckoning :: Few of the GPS devices out now have this capabilit but if you're in an area that is prone to loss of GPS signal, this might be a critical option for you.
Weatherproof :: Where you mount your GPS antenna will impact your need for a weatherproofed unit or not.

Brief Introduction of SiRF and NMEA
Connecting to the GPS Receiver is the first hurdle to overcome, but first a brief introduction. Firstly there are two GPS Standards, NMEA and SiRF. Although SiRF is the up and coming new boy on the block, NMEA is the acquired standard and one used by 95% of GPS applications. Why two standards ? NMEA is getting old, it was designed to talk at 4800 baud (4kbps). Modem speeds today are 56kbps so you can see NMEA is slow. SiRF has brought about that change by increasing speeds and utilising full serial speeds up to 115200kbps. The thing is although NMEA is slow, some GPS Receivers will work at higher than 4800, but this is what NMEA prefers. The amount of GPS data sent down the serial port is very small and can easily be transmitted in 4800 baud so although you could reach higher speeds, there isn't really any necessary to do so. SiRF uses higher speeds because it can transmit the data quickly and put the processor into a wait state which in theory means it utilises less power, newer SiRF chipsets like SiRF IIe also have low power consumption which help even further with sustaining power to the GPS Receiver.