big.al:
1) Laptop spec. isn't terribly important as this isn't terribly intensive computationally. A PII 400 would suffice. You'll need a decent battery life or a cigarette lighter adaptor for the computer, though - unless you're on a boat and have an inverter handy...
2) Quite a bit of software around. If you're planning to use this for navigating while driving then I recommend Garmin's
MapSource product - probably the MetroGuide Europe variant - as this will also be able to download the routes you create into your GPS for when you don't want to drag the laptop around with you. This does the moving map application as well as routing, though you'll need to download the latest update of the web to get the best functionality.
If you want to use scanned maps then have a look at a program from down-under called
OziExplorer.
An alternative to using a laptop is to use a PDA - I guess it depends on how important screen-size/cost is to you. I use an HP
Jornada with my Garmin Legend GPS along with TomTom's
CityMaps software for navigation and OziExplorer CE for using raster maps that I've scanned in.
Hope this helps...
m.