From Microsoft's HALO site ( http://www.microsoft.com/games/PC/halo.aspx#sysreqs ):
System Requirements
* Microsoft Windows 98SE/Me/XP/2000
* PC with 733 MHz equivalent or higher processor
* 128 MB of system RAM
* 1.2 GB available hard disk space
* 8x speed or faster CD-ROM drive
* 32 MB T&L capable video card required
* Sound card, speakers or headphones required for audio
* Microsoft Mouse or compatible pointing device
* 56.6 Kbps or better modem or LAN for online play; broadband to run a server
From the README.txt included in the Dedicated Server installation (yes, I actually installed the Dedicated Server just to get the system requirements..)
Halo Dedicated Server requires a personal computer with the following minimum requirements:
Multimedia personal computer with a 733 MHz or higher processor.
Windows 98 SE, Windows Me, Windows 2000 Service Pack 1, or Windows XP.
128 Megabytes (MB) of RAM.
200 MB of available hard disk space.
Keyboard and Microsoft Mouse or compatible pointing device.
Broadband internet connection.