Hi Hi-Liter!

I have a Wintec WBT-100, which I would rate around 6. It does the job, but it is not very sensitive or particularly fast to fix.

While I don't know for sure, I think that any Bluetooth GPS supporting NMEA output should work fine with the 770.

Regarding software, I highly doubt that the GPS will come with any Nokia 770 software. Your best bet is to install Maemo Mapper, which works quite nicely on the 770. You can also install Roadmap (a 770 version, the latest ones are usually compiled for the 8x0) if you want to use vector maps.