Straight from the Cakewalk website...
What are the system requirements for SONAR?
Windows 98, Windows 98SE, Windows Me, Windows 2000, Windows XP (Note: SONAR is not compatible with Windows 95 or NT), a 400 MHz processor, 64 MB RAM, 100 MB free hard disk space, 800x600 screen resolution/256 colors, CD-ROM drive (for installation), MIDI interface and/or Windows-compatible sound card.
Keep in mind, however, that these are the minimum requirements to run SONAR. If you intend to use SONAR to its fullest (i.e. many audio tracks, multiple real-time effects and DXi soft synths, etc), you’ll want to get a computer with more processing power and RAM, run your display at 1024 x 768, and have a large, fast hard drive.