As matttail said, there appears to something wrong on your particular server, or perhaps there was some error during the creation of your account. Your best option is to contact support (as I believe you have already done).
No, you will start with the full 20GB and this will grow at a rate of 160MB per week. It is a kind of 'loyalty bonus', the longer you stay at DreamHost, the more disk space and bandwidth you get.
Most of the times I've looked, this number is around the 400 figure. However, given the size of DreamHost's customer base (around 200,000 hosted domains), I would consider this normal.
The few times I've had to contact support for general enquiries, the response has been around 24 hours. In the event of a server outage, I use the 'Is the system down?' function of the Web panel and the response is much quicker.
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