(Absolute) Minimum System Requirements

x86:

OS: Windows 98/Windows NT 4.0/BSD/Linux
CPU: 400MHz Pentium II
GPU: Anything with support for OpenGL 1.1 and 16+ MB of VRAM
RAM: 256 Mb
Storage: 2 Gb

PowerPC:

OS: OSX 10.4 Tiger/BSD/Linux
CPU: 400MHz G3
GPU: Anything with support for OpenGL 1.1 and 16+ MB of VRAM
RAM: 256 Mb
Storage: 2 Gb

Coming soon:

SPARC:

OS: Solaris 8/BSD/Linux
CPU: 400MHz UltraSPARC II
GPU: Anything with support for OpenGL 1.1 and 16+ MB of VRAM
RAM: 256 Mb
Storage: 2 Gb

MIPS:

OS: IRIX 6.5.30
CPU: 300MHz R10000
GPU: High Impact Graphics
RAM: 256 Mb
Storage: 2 Gb

PA-RISC:

OS: HP-UX 11
GPU: Anything with support for OpenGL 1.1 and 16+ MB of VRAM
RAM: 256 Mb
Storage: 2 Gb