I am curious about this also. GPS Navigation is the killer app that is saving the pda and will be the future of state of the art cell phones even as it becomes standard in cars.
To begin with I'm not 100% if this can be done on the cheap.
This summer I tried the cheap solution by trying the $99 Delorme Street Atlas/Earthmate solution.
It worked pretty well on the notebook computer in which you can copy the entire data disk onto the harddrive. The only problem was that it had a significant lag. And after I got off course in Boston/Cambridge I kept missing turns that just passed and then trying to readjust to the city's crooked and one way streets was a nightmare even as I was just a few blocks from my destination.
The other big disadvantage was that its street addresses were off by as much as 50 yards. Thank heavens my neighbors weren't that upset after I kept turning down their drives thinking I was home
None the less this was enough to get me hooked on the gadget.
I bought a $200 axim X30 64 meg pda with bluetooth and wi fi on ebay (with the added advantage that it will play mp3's and videos and do the office/outlook schtick. It has basically "limitless" storage via the sd card.
Delorme's Street Atlas was not directly compatible with the pda. Rather you need the Delorme Street Atlas handheld version. And for good measure I wanted the topos and so went for the Delorme Topo USA (Delorme Topo also will not work with a pda unless Street Atlas handheld is installed). They have a package for $249 including Delorme's blue logger gps receiver.
This worked pretty well although the blue logger is rated by most sites as the worst of the gps receivers (very slow at picking up satellites). It's also not water proof. I put that to further test as the gps unit and pda are separate, and I accidentally sent the blue logger through the washing machine. It's very clean now but definitely doesn't work
So I went back and got a different bluetooth gps unit Globalsat BT-338 SiRF star III Bluetooth GPS Receiver (WAAS Enabled). It is supposed to be the best on the market thanks to the WAAS capability (satellites are supplemented with signals from ground towers). I still have not detected a noticeable imporvement in finding the satellites over the blue logger.
The big advantage of course are that now I have topo maps for the whole country. The cons are that you need to plan out your trip before hand as you need to download the maps you need. And Delorme annoying does not let you make address to address directions via the pda (although you can on the laptop). You have to point to your destinations which is sort of a catch 22. Also Delorme's spoken directions seem to have the lag. The speaker on the pda is not very loud so you may have to pay for a gadget to pipe it through your car radio (thus in effect eliminating the usefulness of the radio for listening to the radio).
Now after my little doing the computerized directions has ballooned from $100 to more than $500, here's some observations:
1. You are probably best off buying a self contained pda/gps such as the garmin ique 3600.
2. A totally dedicated gps is probably the easiest solution for in car stuff although the cost could be $1000. The topos are really nice though on a pda -- especially if you are into geocaching (many gps units now are specifically designed for the hobby!)
3. The new crop of cell phones (including the blackberry) have gps built in.
4. When looking at options you should look for something with removal storage -- notably sd drives.
5. If you want to go the delorme route, go for the $249 blue logger/street atlas handheld/topo package. You can use the street atlas feature on the notebook and even if your notebook doesn't have bluetooth installed you can buy a usb bluetooth adapter for about $20.