Another option (if you don't want to purchase another hard drive and if you have sufficient space on your current hard drive) is to use Disk Utility to partition your hard drive. You would then install Mavericks on the new partition.
For example, a 1 TB drive could be split into two partitions:
Partition #1: Mac OS 10.6.8 (the original Snow Leopard and all your original files) 500 GB
Partition #2 (new): Mac OS 10.9.2 (Mavericks, plus new applications such as LR) 500 GB
Similar to Larry's method, the partition method allows you keep using your older software and use a new operating system on the same computer, but you don't have buy a new hard drive. You'll have to restart the computer each time you want to switch to the other OS (by holding the Option key while restarting).
Of course, before you create a partition you should back up your files first in case something goes wrong during the partitioning process. If you are unsure about the steps for creating a partition, here's an online tutorial
, or you can search the Internet for other resources.