Mac pro used was a 3.2G 8 Core, tested with both tiger and leopard
I usually recommend the most up to date - however, you need to consider direct compatability - ie memory & graphics card (X1900XT works in both)
For example - all my macs are the same spec and generation - which means I can swap graphics cards, drives/os, memory etc with no problems
A recon 2007 macpro will work out cheaper and is just (if not more) reliable as a new 2008 MacPro, as they go thru a more intensive testing process.
The bus speed and RAM is faster on a 2008 MacPro - and the 3.2G processor is 0.2G faster than before.!. While this wont have much impact on playback, you might want to think ahead for future HD video input...
choice is yours.....
S