I don't know why anyone would say that an older Mac Pro would not be good at video editing. They are still the standard edit room platform, as the transition to the new mac pro has just started.
First, which editing software are you going to be based on? Its slightly relevant, though all will work great on an older mac pro given you have upgraded to enough ram. I would boot off of a SSD drive or a pair of SSD drives, they would be even faster off of pcie adaptors. I would upgrade to a higher end GPU from the nvidia family, unless you intend to run final cut x alone, in which case you can replicate the dual firepros of the new mac pro to stay on the same curve as that app gets written to take advantage of that gpu setup. The 2009-2012 mac pros can have the CPUS upgraded, you can get up to 3.47 ghz 12 cores. These machines have a lot of life left in them as editors.
The next issue is i/o speeds for your media, you can set up an internal raid and that would be fine for HD. If you start to work with 4k footage, you will hit bottlenecks there but frankly, everyone is without being connected to expensive fibrechannel type storage in facilities.
Look at barefeats.com. You will find a comparison of how you can set up a classic mac pro to benchmark as high as the most expensive new mac pro. I myself am upgrading my 2009 mac pro to this standard as we speak.