At the moment, your best bet on the N80 is Route 66. Prices range from about £160-£180 with bluetooth GPS receiver or you can buy just the software on CD for cheaper. There's also Navicore, but I don't think it's as good.
TomTom should also be adding support soon and I'd expect prices to be around the same.
I reckon buying the maps for your existing sat nav system will be cheaper though.
At the moment there are huge problems with the N80's bluetooth abilities.
The bluetooth connections are known to drop out randomly. Nokia are aware of the problem and advise that they hope to have it sorted in the next firmware release.
Therefore Satnav is near impossible to use on the N80 at the moment, the GPS reciever will keep disconnecting from the bluetooth connection and it's a real pain in the a... neck.
Route66 was listed as compatible with the N80, but due to ongoing bluetooth problems they no longer support the N80.
I personally tried Wayfinder and had the same problem, had to take it back.