OK, after a lot of thought, and a lot of reading... I went for it.
First, I did not want to upgrade my computer - everything I run is built from source and optimised - xorg e.g. I only built the drivers I needed.
So anyway, I bought a GeForce 7600GS 512MB AGP tonight.
WOW. Flightgear is a new experience!!! - I can run it with all the eye candy and also fly at night!
For info. only:
Old card FX5500 - glxgears reports 3400ish @ 16 bit.
new card 7600GS - glxgears reports 6700ish @ 16 bit.
So, you _can_ run new AGP cards in old AGP slots. But you need to do a bit of research.
The page below I read and reread before looking for a new card - then in the shop I asked the if I could return it if it didn't work - the "yes" made me go for it.
This is well worth a bookmark.