You're running Mac OS X 10.5.8 = Leopard.
Mac OS 10.6 is Snow Leopard and was available on a DVD to upgrade Leopard.
Apple no longer sell Snow Leopard AFAIK, though yiou may find a copy somewhere.
For Mac OS 10.7 (Lion) you need currently to have Snow Leopard installed to purchase the upgrade from the app store.
Lion is not available on a disk from Apple.
Apple will be releasing Lion on a USB stick or similar I believe, but it will be more expensive than an app store download.
Apple no longer sell Snow Leopard AFAIK, though you may find a copy somewhere.
Still available at the Apple online store at this writing:
Even Leopard can be special ordered by phone.
A USB SL + Lion will be sold in the next month for $69 to make the update easier.
I would put Lion on one drive, Snow Leopard on the other.
Maybe use your Leo disk for Lion once you have SL installed, tested, updated, and make sure that works first.
There are a number of articles on Lion issues, updates, compatibility and such.
And for a Mac Pro, there are likely to be more hardware and software issues to look into and good reason to tread lightly.