40GB hard drives are pretty much still a standard as far as manufactured PCs. Most of the time, when ordering, you can upgrade to a 60GB for about 10 bucks. If your going to use your computer for email, Web surfing, and some music- 40GB is plenty.
If you're going to start downloading music- which I do not condone, btw- you need a larger hard drive, say 80GB or 120GB. If you're going to be working with movies, video editing, and music, then I would suggest at least 160GB hard drive.