The fundamental graphics "engines" are the same on workstation and desktop video cards; but the workstation cards definitely have more stable firmware, and are better suited for high-end rendering applications. In addition, as Callandor noted, they have better support and are more rigorously tested. You are, of course, paying for that ... but if you stay with a modest card (the FX580) it's a reasonable value.
The desktop cards are more focused on Direct-X support for gaming, but they do still support OpenGL ... they simply aren't certified and both the firmware and the drivers are optimized for Direct-X (thus better gaming support). They also tend to push the GPU more ... thus consuming higher power and generating more heat (although workstation cards will also stress the GPU when rendering).
I agree a good card of either "flavor" would be fine for your applications ... but for a workstation class system I'd use the workstation card, and given that decision, I'd pay the small upgrade charge to go with the FX580.
... but as I noted in my original comment, you could always get the base card and simply replace the video card if you ever decide you need more performance (or decide you want to do some gaming). If you DO decide to go with a desktop class card, the 9800GT is a superb choice:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130435 I'm not a gamer, but that's what I used for my most recent desktop system [primarily so there's enough gaming performance for my grandkids when they visit :-) ].
