If your architecting work trumps your gaming "work" significantly, then a workstation class GPU should be your priority. It will make a difference when using Autodesk and other CAD/CAM/etc tasks. Both the NVidia Quadro and ATI FireGL/FirePro/FireStream GPUs fit the bill for these applications and they provided "certified" drivers for certain key design applications.
It is also quite true that they will not perform as well in gaming. The HD5850 is a great gaming GPU (especially for the relative cost) and it will run circles (several in fact) around just about any workstation GPU in terms of gaming performance.
As decode points out, the HD5850 will likely perform well for your purposes. It should perform in an acceptable fashion with your work apps and it will defintely out-game the Quadro by leaps and bounds.
I, too, recommend the HD5850 as long as you understand the capabilities and limitations of each. Good luck!