Quadro's can handle for times (hence the name "quadro") as much a workload as the most casual processor can, I guess. But hhmmm... I'm still thinking that a dual 9800GX2 SLI setup MIGHT just be able to come close to the performance of a workstation processor at the same price.
And of course it's gotten more shading and texturing power. Professional-level graphics cards tend to be monsters at high-poly works most of the time. When it comes to shading or texturing, they may lag behind their GeForce counter-parts... and heck, since a SLI 8800Ultra setup can handle about half the amount of workload a Quadro FX 4600 can, I think SLI 9800GX2 can at least handle 3/4 the workload of that Quadro FX 4600.
Not to mention bigger memory capacity for more textures and stuffs, too. Doesn't the Quadro FX 4600 have only 768MB of VRAM? And I think it's based on the G80?
Edit: I know. How does a QX9650 at 4.2GHz and SLI 9800GX2 sound? That'd probably be almost as good as a quad Xeon rig with single FX 4600.