Please be aware that Blizzard have yet to announce any system requirements, I'd guestimate the following are a reasonable figure.

Minimum System Requirements:

• Windows XP or Windows Vista
• 512MB System RAM
• 2 Ghz Intel Pentium 4 or equivalent processor
• 128MB Direct3D compatible video card
• and DirectX 9.0 compatible driver;
• 8x DVD-ROM drive
• 4.5 GB free hard disk space
• DirectX 9.0c (included)
• DirectX 8.1 compatible sound card
• Keyboard, Mouse