I keep all my songs on a 4Gig SD card and have copied over 3Gig of songs to it. If you are reaching the 1000 song limit join tracks together a whole album can be one track although you can't access individual songs. You can use a program such as MediaJoin that enables you to easily compile MP3 files into a single merged audio file.
Is the song limit based on number of files or the average size of an mp3? My nuvi wont read anything bigger than a 4GB card. I can load it up with songs, but they disappear when I put it in the Nuvi. Only the 1st 4GB data appears.