Now you can Install Garmin Mobile XT GPS for your Nokia 5800. if you want to download professional GPS software for your Nokia 5800, Garmin Mobile XT application is designed for use on Symbian smartphone devices running S60 (ie.Nokia 5800, Nokia E6x, Nokia N7x) 3rd Edition OS . according to  garmin news. you must have a previous version of the XT software installed on your NOKIA 5800 SD card to have all of the help files, voice files and basemap files. These are not included with this update. and during installation, you will be prompted to turn on your Garmin GPS.
Please Note: You will not gain access to Garmin Online features and/or the navigation will be unavailable without connecting your Garmin GPS. A minimum card size of 256 MB is recommended for Garmin Mobile XT software with maps. Please save all appropriate data before beginning the update.
Garmin Mobile XT app download link and installation instruction

Tags: Garmin GPS Nokia 5800, Garmin Mobile XT GPS, Nokia 5800 gps software