If you own Rock Band, you can go into the game, go to Options > Export Songs, this allows you to export all your Rock Band (1) songs onto your HDD so that you can play them on RB2, it costs 400 Microsoft points though.
They take up 1.71GB of memory.
I started mine now, (11:35am). I'll tell you when it's finished to give an estimate of how long it shall take.
EDIT: Finished now (11:46am). So thats 11minutes. Not bad.