[g_eidsvig]

well, its really up to you to take the time to try as many possibilities as you can.
for me, I run a GeFroce2 32MB card. and i basically have everything on medium to high settings. and get good fps on 800x600 resolution.

basically, turn everything down to lowest, leave res at what you have it as, a gf4 should be able to handle 800x600 and more.
then increase important things like models and textures to higher settings untill satisfied, if they get to max, then throw in bonuses like volumetric smoke. (dont bother with volumetric lighting, its not that big a difference)
dont know if this helps, but its a start.