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