The Xbox as it ships does not let you load MP3s onto it, probably as part of a deal between Vole and the music biz. However according to this 'ere site, making the machine do what Vole doesn't want you to do is dead easy. All you need is a a hard drive of your choice and a USB 2.0 to ATA adaptor.
After a bit of juggling you can load content to the drive via windows. Plug it into your 360 and go to media and select portable device and it's all there.
DRM is not what it used to be. Like fings. ยต