Unfortunately it seems that the poor floating point performance on AMD processors (except Athlon) severly limits framerate. My friend has an AMD K6-2 350 with 8 Mb Sis6326 card. His framerate has never gone over 20 fps. My computer, (400 Mhz Celeron 8 mb Intel740 card) averages 30 fps when flying level, and has never dropped below 15. (The one time it got to 15 fps was in a bomber formation of 12 b17s.)
I did manage to increase my friends frame rate by a couple fps by using DX 7a, the very latest video card drivers, and turning off ground clutter (cntl-c). (We also tried defragmenting the hard drive, but that didn't seem to help much.)
Solution: Buy an Intel processor, or AMD's Athlon if you want to fly AH.