About the same as doom 3. Just want to add if you have played doom 3 i wouldn't buy quake 4, to me at least they were like the same game just different stories.
Heres the actual specs:
Quake 4 Minimum System Requirements
English version of Windows 2000/XP
Pentium 4 2.0 GHz or Athlon XP 2000+ Processor
512MB RAM
8x speed CD-ROM drive and latest drivers
2.8GB uncompressed hard-drive space, plus 400MB for Windows swap file
100% DirectX 9.0c compatible 16-bit sound card and latest drivers
100% Windows 2000/XP compatible mouse, keyboard and latest drivers
DirectX 9.0c included
3D Hardware Accelerator Card required
100% DirectX 9.0c compatible 64MB
Hardware Accelerated video card and the latest drivers
ATI Radeon 9700
ATI Radeon X300 Series
ATI Radeon X550 Series
ATI Radeon X600 Series
ATI Radeon X700 Series
ATI Radeon X800 Series
ATI Radeon X850 Series
NVIDIA GeForce 3/Ti Series
NVIDIA GeForce 4/Ti Series
NVIDIA GeForce FX Series
NVIDIA GeForce 6 Series
NVIDIA GeForce 7 Series
Multiplayer Requirements
Internet (TCP/IP) and LAN (TCP/IP) Supported
Internet play requires broadband connection and latest drivers
LAN play requires network interfaces card and latest drivers
