I've got 2500 songs on my N900. It works really well ONCE tracker has indexed it all. This can however take quite some time.
Therefore, I suggest you copy your initial large music load to the device in the evening, then get a night's sleep while it indexes and startup the Music Player in the morning (perhaps it only needs 30 minutes or an hour, but indexing it all, extracting album art etc. etc. is quite some work for this small wonderdevice).