Your problem is the processor... and not enough RAM.
You are under the minimum requirements for this game...
I would suggest you to buy + 1GB RAM (or more, depending on your mother board/OS) because it's cheap, and, if you can't buy a new processor (which I would recommend core 2 duo or core 2 quad for GTA IV), you should set the minimum value on the graphics settings for things that use the processor too much.
Settings I would recommend for you: <your chosen resolution> - medium/medium1/1/1/0 <resolution> - textures/filtering, view distance/detail distance/traffic density/shadow density)
Changing resolution on my computer doesn't change absolutely anything on FPS, so I think you should decide which resolution best suits you regardless of performance. You can change textures and filtering as much as you want because they are rendered by the graphics card, but I don't recommend you putting textures higher than "medium" because it will use too much graphics memory and the processor has to load the textures anyway so it would reduce performance. You can increase view distance a bit but when running fast on a car you will notice slowdowns.
If you try increasing traffic density or detail distance you will notice major slowdowns and lag.
If you set shadow density on any value above "0", you'll notice major slowdowns on nighttime.
GTA IV pushes too hard when it comes to processor - having a great graphics card is not enough when using a single core processor. While a 8600GT (with enough memory) is enough for playing it on medium/high graphics, gta iv needs at minimum a really good dual core for playing. (even though minimum requirements are core 2 duo, and recommended is core 2 quad).
The 9400GT isn't really a good card anyway, even though it has 1GB of memory your not really going to use that considering the power of the card.
More RAM doesn't equal speed, despite what a lot of people think.
Also, to the Crysis FPS questions.. Crysis is a terribly optimised game. Even the first versions of Far Cry 2 ran better than Crysis, they also look much better too.
I've completed the game today. I've selected the revenge branch and in result Kate was killed on the wedding. After I have completed all the missions and titles rolled. I was cruising around town and had a call form an Unknown caller... When I picked it up it was Kate proposing to meet.
Today when I was going to meet Little Jacob (he called me), I went to his place, stopped the car and waited for him to get in the car.. he was coming, and when he was about to open the car door, *CRASH*
*Your friend has been injured*
Another car hit him so hard that my car had blood splatted in the whole body, and Jacob flew away about 30 metres
I lol'd
The other driver tried to escape just after that, but I managed to kill him.