For anyone who is building or purchasing a computer and wishes to ensure the best performance from ZBrush 2, here are some guidelines:
RAM is probably the most important factor, as it has to do with how much ZBrush can do before it needs to access the virtual memory file (which is MUCH slower). More RAM means larger meshes and larger documents can be handled without accessing virtual memory.
CPU speed is important to all ZBrush operations, including painting, mesh editing and rendering. Multiprocessor systems or hyperthreading CPU’s will give a performance boost.
Hard disk speed comes into play whenever you exceed what your RAM can handle. The faster your disk access, the faster ZBrush can write and read the virtual memory.
ZBrush utilizes its own software-based rendering-engine rather than hardware-rendering. As such, your video card does not matter much so long as it can display at least 1024x768 resolution in 32 bit color (True Color). It is therefore best to select your card based upon the needs of your other software rather than ZBrush’s needs.