Highly Recommended:
OS: Windows Vista/Windows 7/Windows 8 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)

Note About Graphics Cards:
ZBrush is software rendered, meaning that ZBrush itself is doing the rendering rather than the GPU. Your choice of GPU will not matter so long as it supports the recommended monitor resolution.