I upgraded last April, but before that I played AH on this system from beta forward:
Gateway Pentium Pro 200mhz
128 RAM
Creative Labs Graphics Blaster 16meg
Cable modem
AH worked just fine for me, I saw nothing wrong with my average 15-25 FPS. When I got near badly damaged fields (smoke), it would sometimes drop to 7-9 FPS and that was a little harsh. Clouds never bothered me much. Was just happy to be able to play.