You can still use itunes if you want, but it is better to use plain old windows explorer (i.e. drag n drop). itunes gives your music a weird directory structure ( folders called F00 F01 etc. with files ADF.mp3 etc. )
using drag n drop will make it easier to figure out what music is on the device when it is connect to your PC. This method also lets you choose between filebrowser and Database, I personally prefer the filebrowser, that way I don't have to make sure all my tags for my 3000+ songs are correct.
You can also use a folder synchronization tool to sync your music folder on your PC to the music folder on you device. I used to use a program called syncback while I was using windoze. (I'm on linux now and use krusader if anyone wants to know).
hope this helps you

p.s. reading the manual is a great way to answer questions like these and discover new things that rockbox can do