Highly Recommended:
OS: Windows Vista/Windows 7 32-bit/64-bit
CPU: Pentium D or newer (or equivalent such as AMD Athlon 64 X2 or newer) with optional multithreading or hyperthreading capabilities.
RAM: 2048MB required for working with multi-million-polys. 6 GB recommended. (ZBrush 4 is a 32-bit application, but can use up to 4 GB of system RAM)
HDD: 16GB of hard drive free-space for the scratch disk.
Pen Tablet: Wacom or Wacom compatible.
Monitor: 1280x1024 monitor resolution or higher (32 bits)

Minimum System Requirements:
OS: Windows Vista, or newer, 32-bits or 64-bits.
CPU: P4 or AMD Opteron or Athlon64 Processor (Must have SSE2 : Streaming SIMD Extensions 2)
RAM: 1024MB (2048 MB recommended)
Pen Tablet: Wacom or Wacom compatible.
Monitor: 1280x1024 monitor resolution (32 bits)