I'm on a 1.8 Ghz, 512 RAM, 9800Pro. That's below minimum, as far as the RAM is concerned. The game runs flawlessly and fluidly, except for some acceptable FPS bog down in port and a more severe one at very high time compression (>6000).
What I did though to achieve that was tuning different settings in the drivers panel from "quality" to "performance" or "high performance". Environmental effects off as well as that bloom thingy. Also texture quality low (thank god there is a numerical read out for the sonar bearing!
). Before doing these things, the game ran like crap. What I also did was disabling FSAA in drivers pannel - dunno if that also had an influence, just figured when the game doesn't support it, I better turn it off by default to avoid possible conflicts.
Game looks still better than SHI, which is good enough for now.
In fact, it looks still pretty darn good and better than SHIII imho. And I actually like the post processing filter - initially, I was afraid it would suck from looking at some screenshots.