A followup to my original post -- I did a lot more testing, and paid careful attention to how each card behaved with each version of the driver I tested.
The problem appears to be related to an interaction between the NVidia driver version and the card chipset, and does not appear to have anything to do with PCI-Express (which I determined by testing several AGP cards and seeing the same problem on all of those 6000-series AGP cards, see below).
I am still seeing very poor frames/sec (i.e., 4 fps or much lower) when I run FlightGear on NVidia 6200TC AGP, 6600 AGP, and 6800Ultra PCIe&AGP cards I've tested with drivers after 6634. Note that 6634 (which is no longer available for download on the NVidia FTP site) was the latest driver from NVidia that gave me decent performance on FreeBSD (i.e., > 30 frames/sec), and I carefully tested all drivers more recent than 6634 available to me (e.g., 7174, 7667, and 7676). Performance on 7174 was the worst at less than 1 frame/sec, and 7667 and 7676 were slightly better at 3-4 frames/sec. Note that I am NOT seeing performance change between driver versions on any 5000-series cards I've tested (in particular, a FX5200, which yields roughly the same performance on all driver versions tested).
My current theory is that something changed in the NVidia Linux/FreeBSD driver after 6634 such that FlightGear performance went into the toilet on at least FreeBSD, and it's still a problem when using the very latest NVidia driver available.
I would LOVE to hear from anybody who is running FlightGear on Linux (any version) and getting acceptable smoothness (>=30 frames/sec) on any NVidia 6000-series card (e.g., 6200, 6600, 6800, AGP -OR- PCIe) using driver version LATER THAN 6634 (e.g., 7174, 7667, or 7676).
I will post a new thread regarding FlightGear performance under FreeBSD when running driver later than 6634.
P.S.: For those of you who haven't heard about FlightGear ( http://www.flightgear.org/
), it's an open source Flight Simulator program, and is surprisingly good for an open-source package. My hat is off to Curt Olson and crew who continue to develop it. It's very cool to see and be able to modify C++ source code that does a ton of practical 3D manipulation.