Halo Xbox Requirements:
Xbox Game Console
Halo PC Requirements:
CPU: 733 MHz
HDD Space: 1.2GB
Graphics Card: 32MB / 3D T&L capable, GeForce 2+ recommended
Halo PC Suggested Requirements:
HDD Space: 1.5GB, with an additional 3MB per screenshot, plus approximately 4.6MB for each patch downloaded
Graphics Card: A GeForce4 Ti 4200 or equivalent will let you play smoothly at 800x600 with Pixel Shaders. GeForce4 MX cards are not recommended: their "4" is a misnomer.
Halo Mac Requirements:
Currently, the minimum system will be a 700 MHz G4, with at least 256 MB of RAM, and a Radeon 8500 or GeForce 2 MX with 32 MB VRAM. Please note that these are not final and are likely to change at anytime. But, it should give you an idea of what the specs will be like.