Following up on Thomas' post on how to create a site collection in a specific content database:
You can also use the STSADM command CreateSiteInNewDB
However this does not quite cut it. You would like to decide on which content database each time. To do that, you could use stsadm extensions. It will require some codewriting and testing, though. Luckily, a MVP, Gary Lapointe, has already made it for you. A "Create site in database" is available in his stsadm custom extension kit:
There are also other STSADM extensions available. If you have used or made one you would like to share, please send me an mail.