Whatever you do, don't take the manufacturer's word for it. Hard drives hold information, and whether it is a 20 GB Apple iPod, a 20 GB Dell DJ, or a 20 GB Sony Walkman - they all hold 20 GB of info. None holds more than the other.
That's sound advice. How many songs your player will hold depends entirely on you, what kind of music you prefer and, most importantly, the method of compression you choose and how much compression you are willing to tolerate.