Addressing the title, the two cards you listed are not in the same category. ATI has the 4870 for the consumer market and the Fire GL (line purchased from Diamond) for the workstation market. nVidia ATI has the GT 200 series for the consumer market and the Quadro for the workstation market.
As for the question, are we talking Mac like graphics design or AutoCAD / 3D Max rendering ? If we talking about rendering building exteriors / interiors from wire frame models, a $1,000 or better Quadro should be considered the entry point. If we talking, magazine layouts and photos and such, I don't practic ein that field, but I'd lean towards the ATI 4870 series or the nVidia GTX 200 series at the moment. If ya not in a rush new cards shoukd start arriving from ATI within a month and Nvidia maybe 1 or 2 months behind. Personally I wait aboy 406 months after an introduction of a new series. They don't call it the "bleeding edge" for nothing
Jack, you totally didn't answer or even address his question. He's asking if a low end professional card is better for rendering than a high end consumer card.
I don't know the answer personally, but i can't imagine that a $200 workstation card will even come close to the performance of a $200 gaming card. The workstation cards are just so underpowered at that price range that i doubt drivers would make any difference.
That said, if you're not doing any 3d modelling or rendering, you don't need a high end card anyway. Graphic design programs like photoshop and quark don't rely on GFX performance - they rely on the components in this order: CPU -> ram size -> HDD performance -> ram speed -> THEN gfx performance. Most graphic designers would likely be able to get away with low end cards like a 9400gt or HD4350, unless you have more than 30 or 40 hi-res (100+ mb) files open at one time.
Depends on the application, some benefit greatly from workstation drivers (usually OpenGL based), others don't (usually Direct3D based).
It's really application dependent, where even a low end Quadro or FreGL/FirePro card can outperform the top end consumer/gaming cards, although the inverse is rarer but you might find a $100 desktop card outperform a low/mid-range workstation card that costs twice as much as the most expensive consumer/gaming card.