you should be OK to upgrade
just folow the steps
Snow Leopard first - get DVD from online apple store
Lion via Store App
Montain Lion via Store App
You should do some reading on the migrations for each step.
And a lot of owners of the 2008 have trouble, power and startup issues, and seems that 10.8.x ML is not and does not run well. And going back to 10.6.8 which does and all the problems? they go away.
Yes you can, but you may not want to or regret.
And do keep the old system as is.
Do a clean install on a new drive that you format for just Lion and ML.
Keep your working system.
Well, all you need to do is to look at what type of hardware components are used in your Mac Pro and that will let you know which…oh wait, no, no that won't help at all. Since Apple has been using the same hardware and components for something like four or five years. It must be nice to be able to sell a 2008 computer in 2013 and still charge more for it than your competitors. I suppose that companies such as Honda and Toyota should still be selling their 2008 model cars, trucks, and SUV's and selling them for more than their competitors who are actually manufacturing and selling automobiles with the newest advancements and technologies. I suppose that this is to be expected in what Steve Jobs described as the post PC era.
Sorry, but I needed to do a little venting.
tzbikowskihas the right steps, but there are a few more twists and turns.
10.6 does indeed come only from the Apple online store, but you have to CALL them on the telephone and ask to buy the 10.6 DVD, as it is "put away" and you cannot just click and buy it.
You will have to CALL again to get 10.7 if you wish to Install 10.7, and purchase a download code and special instructions for download, as that version is also "put away".
10.7 BREAKS THINGS. the top 6 are:
Quicken 2007 * (but there is a US$15 upgrade available)
Adobe PhotoShop CS2
Word and Office 2004
FileMaker Pro 6