For some truly astronomical figures, check 'The Diverse and Exploding Digital Universe', an annual white paper by IDC, sponsored by EMC. According to their estimates, the digital universe in 2007 was 281 exabytes (281 billion gigabytes). In 2011 it is projected to have grown to 1.8 zettabytes (1800 exabytes).
One rather scary point from the study is worth contemplating:
Of that portion of the digital universe created by individuals, less than half can be accounted for by user activities - pictures taken, phone calls made, emails sent - while the rest constitutes a digital "shadow" - surveillance photos, Web search histories, financial transaction journals, mailing lists, and so on.
In other words, the trail we leave involuntarily in the digital universe - information about us - is now larger than the information we deliberately create.