I don't know anything about pro tools. But if you aren't doing anything visual and graffic intensive... why the need for dual CPU systems and video cards?
Sounds like you would just need a decent amount of RAM and hard drive space. A solid CPU as well, but I'm not sure why you would need dual CPU systems, that's some pretty hardcore processing.
Like I said, I don't know pro tools to know how much that really needs, but I always just assumed you need the massive power for video editing, wasn't sure about audio editing.
Part of me says just go with the cheaper option of those three except thats a 2009 model, and yours is 2008.... so whats the point really? Might as well get something newer. So the middle option seems the best in that case, a bit newer. All three have good specs so it seems the graphics card is one of the major differences between the three.