can I keep my Leopard install? or will I have to fully reformat the drive?
You can keep your Leopard install and make another partition on the disk for Tiger. You should back up your data first in case you make an error.
Use Disk Utility and follow these directions (you can also view them from the online Help system.
To create new volumes on a disk:
1. Select the disk from the list at the left, and click Partition.
2. Select an existing volume in the Volume Scheme list, and click Add (+).
3. Disk Utility splits the volume into two, leaving the data in one of the new volumes. If the volume is less than half-full, Disk Utility creates two volumes of equal size. Otherwise. it creates one volume large enough for the data, and another volume with the remaining space.
4. Resize the volumes as needed.
You can drag the dividers between the volumes in the Volume Scheme list, or you can select a volume in the Volume Scheme list, and then enter a value in the Size field.
5. Choose how to format the volumes that will be erased or created.
6. For each new volume, select it, enter a name, and then choose a format.
7. Click Apply.