The "real" answer for CAD/DCC/etc work is a workstation card, either Quadro or FirePro. Given that you're looking at a midrange-ish AMD system, though, I'm going to go out on a limb and say the asking price of those ($400+ for what you'd want) will be a little steep.
With that in mind, I'd second GeorgeH's advice - get a decent nVidia card. From what I hear, the Linux drivers are better, and there's a better chance of GPGPU support with them in general, either by default or with minor work (for example, Adobe Premier, where it's quite easy to enable the GPU MPE for all nVidia cards).
Without some idea of your budget and what exactly you're connecting, it's a little hard to give exact recommendations, but something from nVidia in the $100-$200 bracket, with a pair of DVI-D ports, a DP output, and at least 1GB RAM, is probably going to work well, last a good long while, and not break the bank too badly.
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