Ahkay, after seeing
this message i must tell you all something important.
If you want to play GTA4 on at least lowest details, you need the configurations below.
Lowest on 800x600:
Processor: Intel Pentium DualCore E2140 1.6 GHz
Graphic Card: ATI Radeon HD 2400 256 MB
RAM: 2 GB 800 MHz DDR2 or 4 GB 667 MHz DDR2
Hard Disk: Minimum 15 GB of free space on 40 GB HDD.
Medium on 800x600:
Processor: Intel Core2 Duo E8400 3 GHz
Graphic Card: Nvidia Geforce GTX260 896 MB
RAM: 3 GB 1333 DDR3 or 2 GB 1066 MHz DDR2
Hard Disk: As Above, but on 160 GB HDD
Highest on 1024x768
Processor: Intel Core i7 920 2,66 GHz Overclocked to 4 GHz
Graphic Card: Nvidia GeForce GTX295 1792 MB
RAM: 4 GB DDR3 2000 MHz or 4 GB DDR3 1600 MHz
Hard Disk: What do you think? Configs like that have at least 1 TB HDD
(that's some nice shit ;])
Hope it will be helpful.