GTA 4 Lagging

Hello, somewhy GTA 4 is lagging, my specs should be more then the required.

I'm using:
Motherboard - Asus P5N-E SLI (Nvidia nForce 650i)
Processor - Intel Core 2 Quad Q9505 2.83 GhZ, 1333 MhZ FSB
Memory - 4 * 1GB Kingston PC2 6400 HyperX 2
Video Card - Nvidia GeForce 9800 GT 1Gb ram

Win XP SP3

I've killed all unnecessary processes and the system is currently using only 890 MB of the 4GB available.

I'm using a 30" Dell display and am trying to run GTA 4 at 1920 * 1200 resolution with high settings, is this too high for my system? what are the recomended settings for me?

thanks.