On my trip to Europe I forgot to pack my cassettes (yes, it was that long ago) and I was on a tight budget so I only had the tape that was in my walkman. One album on one side and another on the other side.
So I think you'll be fine with 400 songs.
My suggestion would be to grab some books on tape (MP3 format obviously) or comedy MP3s. Just for when you want to have a different source of entertainment. They only have to me 32 kbps MP3s, so a 50 minute comedy bit will take up as much space as one song that's recorded at 320kbps.