Hi Saad,

for bike and offroad application there are IMHO two choices: a GPS-Receiver like the Garmin or Magellan stuff or a PDA with MIL standard.
Both are expensive.
You wouldn't find free maps for Magellan or Garmin on the internet, but several sites with waypoints/tracks from other useres.
A MIL PDA is quite more expensive, but you are able to use a lot of different software and (espacially for the USA) map data.

If you want to use a standard PDA, beware the high charge against vibration on bicycle use. Perhaps a good Outdoor-box for the PDA will do it's work, but I can't promise you ...

Greetings,
André