Greentooth, so long as they're not protected, you should be able to use iTunesAgent to do it.
@ Angel, it doesn't actually put any part of iTunes on your phone. All it does is treat your phone like a thumb drive or something, and places the music on there. The reason it's a good app is cause it's pretty automated, and you can setup a Smart Playlist that changes all the time.
You may also want to check out Winamp. I recently switched to using that for my iPod and entire media collection (video AND music) and it's freakin awesome. ALOT more light-weight RAM-wise than iTunes, and it syncs just as good. It's not quite as user-friendly, but way more robust, with Shoutcast and so many options for internet tv and video and whatnot.
If you choose to do that, I'd suggest ditching the built-in iPod syncing and getting MLpod. It works awesome, and will natively sync your N-series phone, as well. When you first plug it in it will ask if you want to treat it like an MP3 player, and you can set autofill preferences, so when you plug it in, it will erase what music is on there and replenish it with a new set, based around a percentage of total capacity. Thus if you said 50% autofill on a 2GB card, it would automatically toss 1GB of music on there for you.