The box fan was a bit of hyperbole.  The inside of the case is otherwise quite cool.  The 6990 vents out from both ends, so I've cleared paths each way, removing one drive bay and moving an egress fan there as well to maintain lower air pressure for that path, although a fan venting *out* through a filter/mesh goes against all the rules of thumb over the years.

I don't have a reliable way of measuring flowing air temperature at the moment so I can't capably determine the temperature differential between the card's intake (in the center of the card) and its dual exhaust.  I'll tell the wife this is the time of year to have an HVAC consultant look at our furnace and guide him over to my computers as well. :-)

I'm about this >< close to buying a watercooling kit and waterblock for the 6990 and seeing if it solves the problem.  I'm coming around to the idea that I have a defective card, but after removing and reseating/pasting the heat sink I've voided the warranty.  This would also eliminate the need for an extender so I could put my 5870 in this box as well.

Watercooling kit and lines + waterblock + 6990 will go fast on craigslist to some gamer when I'm done with this thing and I'm throwing distance from the Cal campus. :-)  (Jeez, I just now realized what those holes with gaskets are for on the back of the case...)