I've made this topic because we're getting a lot of people asking us whether or not they'll be able to run GTA IV on their PC, and if not, what should they get, etc.
No official requirements have been released yet, but by looking at other recent games we (or rather chris82) has produced this spec list - I think it's probably about halfway between the minimum and recommended specs.
Windows 2000/XP/Vista
Intel Core2Duo E4600 / AMD Athlon 64 X2 6000+
1GB of RAM (Windows 2000/XP)/2GB of RAM (Windows Vista)
nVidia 8xxx series/ATi 28xx/38xx series
UPDATE: 30th October 2008 - Official specs now released, topic has been updated, see below for specs.
Minimum System Requirements
OS: Windows Vista - Service Pack 1 / XP - Service Pack 3
Processor: Intel Core 2 Duo 1.8Ghz, AMD Athlon X2 64 2.4Ghz
Memory: 1.5GB, 16GB Free Hard Drive Space
Video Card: 256MB NVIDIA 7900 / 256MB ATI X1900
Recommended System Requirements
OS: Windows Vista - Service Pack 1 / XP - Service Pack 3
Processor: Intel Core 2 Quad 2.4Ghz, AMD Phenom X3 2.1Ghz
Memory: 2 GB (Windows XP) 2.5 GB (Windows Vista)
18 GB Free Hard Drive Space
Video Card: 512MB NVIDIA 8600 / 512MB ATI 3870
So if you're not sure you'll be able to run the game, just post your specs here and I or someone else who knows what they're talking about will answer you, and if necessary tell you what you should get in order to run the game well.
You might also want to post your monitor resolution that you'll be planning on playing at.