What we currently know about the SC2 system requirements:

• Simultaneous release for Windows XP, Windows Vista, and Mac OS X
• Will use DirectX 9 and require Pixel shader 2.0
• Will be fully compatible with DirectX 10; no word on if DirectX 10-specific effects will be used
• Mac version will use OpenGL

More specific system requirements have yet to be officially released by Blizzard as of yet. However, they have this to say about what we can expect...

"For Starcraft II, we wanted to maximize compatibility with less capable systems to ensure hassle-free game play for as broad a player base as possible. Yet we also wanted to utilize the full potential of any available hardware to ensure the game’s looks were competitive. This meant supporting a wide range of hardware, from ATI Radeon 9800/NVIDIA GeForce FX’s to the ATI Radeon HD 4800s and NVIDIA GeForce G200s"