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4.0  System Requirements
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Age of Empire's III minimum system requirements are:
-Microsoft Windows XP
-A computer that has a 1.4 gigahertz (GHz) processor
-256 megabytes (MB) of system RAM
-A hard disk that has at least 2 gigabytes (GB) of available space
-A 32x speed CD drive
-A 64 MB video card that supports Hardware Transform and Lighting
-An audio card and speakers or headphones
-A Microsoft mouse or a compatible pointing device
-A 56.6 kilobytes per second (KBps) or faster Internet connection for online 
play
-DirectX 9.0c

Source: Microsoft
http://support.microsoft.com/kb/907884