* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...
Yes you can, within limits. For example, if with your Mac Pro you wanted to upgrade to Lion you would have to install Snow Leopard first because the Lion download requires SL. On the other hand while it has been debated about Snow Leopard licensing requiring you own a copy of Leopard to upgrade, many have simply jumped from 10.4 to 10.6 in a single step.
Internal updates such as 10.4.2 to 10.4.11 (vs. upgrades such as 10.4-->10.5) are often done with "combo" (combination) updaters which jump from one sub version to the latest also in one step.