bac522 answered your questions, but I think it's important not to misrepresent Maemo Mapper as the end-all-be-all solution. It's not the most user-friendly interface - it requires some extra work on the part of the user. Specifically, you have to be connected to the internet to use many of its features (map auto-download, routing), OR you have to download in advance all of the data (maps, routes) that you plan to use. Also, the POI database is initially empty - you have to add your own POI data or copy one from someone else.

Maemo Mapper is completely free, but the 770 and N800 don't have GPS hardware, so you have to purchase a separate bluetooth-enabled GPS receiver ($60+). Navicore is another option; it comes with a separate 770/N800 GPS addon and costs significantly more than the price of a GPS receiver on its own, but you get a lot of additional features that you can't get from Maemo Mapper - things like pre-downloaded, vector-based maps and the ability to generate routes without an internet connection.

Basically, you get what you pay for. It's a bit like the difference between Linux and Windows - the latter costs more and is easier on the end-user, but Linux is free and can be quite usable if you know what you're doing and/or have the time and motivation to learn.

If you already have a bluetooth-enabled GPS receiver, then it's easy to justify the little bit of extra work that Maemo Mapper requires in order to utilize the device that you already own. In that case, just give Maemo Mapper a try, and if you need something a little more end-user-friendly, buy the Navicore kit.

If you don't have a GPS receiver, then your choice is more difficult: buy a cheap standalone bluetooth-enabled GPS receiver and use Maemo Mapper, or buy the more expensive Navicore kit. In this case, you can run Maemo Mapper without a GPS receiver, so if you're on the fence, you can just install it and poke around the user interface (and the help file) to see if you're willing to cope with its limitations.

Maemo Mapper is free. I obviously don't make any money from it, so I have no reason to push you to use it. It is much better (for both of us) if you choose the solution that works best for you.