Not sure if your computer is up to the task? For some weeks now we've had a thread on the forums for just that. Will my PC run GTA IV? Here you can post your specs and read our own best estimations at the specs, and some of our more knowledgeable members will do their best to advise on any upgrades we think you may need.
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
EDIT: The information appears to have been removed from the Games for Windows site. The reason for this is currently unknown.