Forum OpenACS Q&A: Re: Is Postgres/MySQL ready for VLDB solutions?
Dynamic database-driven websites tend not to accumulate gigabytes of new data per week. You're not likely to find much practical experience-driven advice here on database systems of that size, and since we support Oracle as well as Postgres with our toolkit we often see our largest, most dynamic sites implemented using Oracle as the back-end.
But there are people in the Postgres community proper who have experience with datasets that grow as rapidly as you describe, so I'm quite sure you'll get some useful info if you ask there . Go to postgresql.org and click on the "lists" link - you have to subscribe to post unfortunately but that's life!