Upgrading from 10.5 to 10.6...

I've found a lot of great info about upgrading my Mac Pro Workstation from 10.5 to 10.6, but have not found an answer to a specific question I have.

- I want to upgrade my install from 10.5 to 10.6.

- I have the 10.6 DVD's from Apple.

- Does the install ' blow away' my hard drive with a fresh 10.6 install, or does it ' upgrade' my existing install? ( i.e., maintaining my apps, etc..)

This is for my studio workstation and I need to get it right the first time just with the crazy workload I have...

Thanks for any info!