Source: http://www.adobe.com/products/flash-media-encoder/tech-specs.html
Windows
933MHz Intel® Pentium® 3 (or compatible) or faster processor
For H.264 and AAC support: 1.67GHz Intel Core™2 Duo processor required (3GHz Intel Core 2 Duo recommended)
For multiple output streams: 3GHz Intel Core 2 Duo processor and 2GB of RAM required (quad- or 8-core machine with 3GB of RAM recommended)
Microsoft® Windows® XP with Service Pack 2 or 3; Windows Vista® Business, Ultimate, or Enterprise; Windows Server® 2003 (32-bit) Web Edition; or Windows 7
256MB of RAM (1GB recommended)
40MB of available hard-disk space (excludes archived FLV and F4V files)
1024x768 screen resolution with 32-bit video card
Microsoft DirectShow compatible video capture device
Microsoft DirectX End-User Runtime version 9.0c
Mac OS
2GHz Intel Core 2 Duo or faster processor
Mac OS X v10.5.6
1GB of RAM
50MB of available hard-disk space (excludes archived FLV and F4V files)
1024x768 screen resolution
QuickTime compatible audio/video capture devices
QuickTime 7 software or later