I bought myself a 30GB spare unit for several reasons:

My current collection of ripped MP3s is hovering around 20GB, so a 30GB unit gives me some headroom. By the time I'm getting full, I'd want to buy myself an IBM 48GB or (if available/affordable) a 60GB disk to get loads of future capacity. You mention that your current Empeg is only 20GB. I'd say buy the 30GB now, and then when you need more space, see what hard drive prices are and do a self-upgrade.