I've found that a combination of my 7600GT video card and 2 GIGs of RAM allows my mid range rig to play SH4 smoothly at the highest graphical settings. I get between 26fps and 40fps on the external shots, depending on how busy the environment is. I have the event camera and volumetric fog disabled.
I did notice when I tried the 'Midway' mission one time that my fps dropped to about 17 on the external shots. I won't be trying that mission again.