I was playing with every bit of my hardware at stock speed. All I could really feel was the frames dragging when driving a vehicle at high speeds. That's about it. Stuttering was never a part of the game.
And go through it with a good amount of patience. Try out some of the tweaks mentioned in that thread. A lot of people have had satisfactory results with it. Also keep shuffling your graphic settings. Add the command line text file. The game doesn't scale well so don't & I repeat DON'T go checking FRAPS for the frame rates every now & then. If you're able to play it then keep playing. That's what I did. The trick is to let loose of the frame check complexity built in us. Throw that out if you're playing GTA IV.
If you're on Catalyst 8.12, try reverting to Catalyst 8.11. For me 8.11 gave a bit smoother gaming as opposed to 8.12. Defragment should be done, like everyone else mentioned. Don't keep too many background programs running.
it may not be a bad port but u have to under stand that this game features NO LOADING hence it need to swap data from one plce to another hence a faster ram is a must.....i got 8 fps just by oc-ing ram ........... i see u have 800 mhz ram ...tighten its timings ... oc it ; oc processor.......... oc video card for the heck of it ........
But in game performance is not much different. Game is okay while not driving. However driving is full of stuttering at each turn and dips below 10 FPS constantly.