The Games for Windows website has opened a section for the PC version of GTA IV, which reveals the recommend system requirements:
OS: Windows XP SP2
Processor: Dual core processor (Intel Pentium D or better)
Hard Drive: 18GB free hard disk space
Video Card: 512MB Direct3D 10 compatible video card or Direct3D 9 card compatible with Shader
Drive: DVD-ROM dual-layer drive
The minimum requirements are not known yet.
In the meantime, the information has been removed from the website.