As far as I know, there is no FireGL version of the X1950XTX. ATI's OpenGL offerings stretch from the V3000s to the V7000 series. I believe the highest end version available is the V7350, which is essentially an OpenGL supporting, pimped out X1900, and it costs around US$2000.
If you are in a workstation environment, using applications such as CAD, then you will want a workstation card: From ATi, their FireGL range, and from Nvidia, their Quadro range.
From a gaming perspective, these cards are simply heavily overpriced versions of their non-OpenGL counterparts. If you are after the best gaming card available today that's compatible with the DQ6, I'd say your best bet would be the Nvidia 7950GX2. If you feel like leaning towards the Canadian team, then it would be the X1950XTX. That said, the 8000 series from Nvidia is becoming available very soon (less than a month, I believe), and you would be well advised to wait, if you are seeking the best of the best.