My Celeron-466 was pretty close to DI's recommended minimum, the only difference being a Creative 3D Blaster Banshee and the sim was very playable on it.
In 1999 I was able to run the sim with this level of detail on a Celeron-466 233Mhz with no apparent slowdown! I had to turn some of the detail and distance sliders down, but put micro-texture on FULL to get the great sensation of speed at low level. The resolution was set to 800 x 600 as 1024 x 768 made the HUD too hard to read on a 15" monitor. Jerkiness was visible, but tolerable while on the carrier deck, but in the air things smoothed out. We also tried the sim on a P350 with the same graphics card and 256 meg of RAM. Running at 1024 x 768 with ALL the graphics options maxed out the sim was as smooth as silk.
On gigaherz plus machines with a decent graphics card the sim would run as smooth as silk. My AMD Athlon XP2200+didn't even breathe hard.
The problem might be with the ATI card? Perhaps a Flight Sim Museum reader might have an answer?