These are supposed to be the specs for D3, but don't hold me to it or your breath. But they don't look too terrible IF correct.
Operating System: Windows XP/ Vista
Processor: 1.4GHz or higher CPU.
RAM: 1Gb or more.
Video: 128Mb video card Dx9
Hard Drive Space: 4-8GB
Mac:
Operating System: MacOS X 10.3 (Panther) or better.
Processor:1.6GHz or higher.
RAM: 1Gb or higher, DDR RAM recommended.
Video: 128mb ATI or NVIDIA video card
Hard Drive Space: 4-8 GB