I have a 512mb Nvidia GeForce 8400 GS GTA IV (in lowest graphics setting) still runs a little shaky?

Upgrade your graphics card to at

least hd 4670.8400gs

is terribly slow card

for gaming.very very

minimum would be

hd 4650 for $50.

Try the latest patch for

GTA 4.

You bought a game even though you don't meet the system requirements. You should have known what to expect.

GTA IV requires a 7900, which was a high end card of the 7 series. The 8400 GS is an entry level business card from the 8 series and is not designed for games. The 7900 is much faster than that.

Upgrade your RAM and get a better graphics card (an nvidia geforce 9600GT would be perfect for your system and they're pretty cheap now).

gta 4 has problems even with high end pcs but you should upgrade your pc. get core 2 duo. 4 gb ram. 9800 gt video card.