ok... wow, i'm not sure many of us who tweak our radeon cards even know 1/3 of the options they have, lol.... but fwiw...
(if it's not listed by me, my card doesn't have that option, and you just might have to fiddle around, one thing at a time, to see the effects, unless of course skuzzy can step in with some enlightment, which i hope he does.)
W-Buffer: i have it enabled, but dunno if AH supports it.
32bit Z-Buffer: enabled, but... ??
Alpha Dithering: i have it on error diffusion, i've tried the other methods and couldn't really tell any difference.
Alternate Pixel Center: mine's disabled under the tweaker recommendation.
Anisotropic filtering: disabled by me, i notice i seem to lose fps if i have it enabled. could just be my slow system, tho'
Texture Pref/Mip Mapping detail: i have mine on setting 0 (high quality), i notice i lose FPS, but gain quality with higher settings... again my slow system, so YMMV.
Wait for V-Sync: enabled.. suggested by Skuzzy to be enabled. it ties in monitor refresh and card rates, keeps them in sync, so to speak.
TruForm: my card doesn't support it.
Skuzzy - what kind of Fog does AH utilize? i have 3 kinds disabled and fog doesn't appear... i find a gain an fps or 2 without it enabled.
here's my specs (go ahead and laugh, you know you wanna

)
333 PII
128mb RAM
66mz sysbus
DX9
win98SE
ati radeon 64mb DDR Vivo (almost like a radeon 7200 - what allows my system to even be able to play AH - really gotta upgrade this summer; i get between 2-40 fps depending on situation. avg around 20-30)
1024*768 desktop/game res, 32-bit color.
my system is low end for AH, and everytime HTC does a newer version, i generally start to lose fps, but can opften gain back a little by putzing around with things, but it's a net loss each time overall. so far have managed to maintain minimal playability. i don't think it'll last for much longer, tho...