What is the computer hardware and software used in special effects ?

Silicon Graphics for the most part. Much is proprietary though there are several architectures including RISC and Intel. OS support depends on the architecture. The RISC boxes supported SGI's IRIX OS and the Intel boxes ran NT and Linux. RAM was generally limited to 4GB though I believe the Onyx series supported up to 8GB. Hard drive space was limited only by your budget.