reFuse currently works on Macintosh PPC's only, running OS 8.1 - 9.2.2 (OS X is not supported). Obviously a copy of Pro Tools and Reason would be a good idea. Any computer that can already run Pro Tools and Reason simultaneously should have no problem accomodating reFuse as well.
OMS 2.3.8 is also necessary, to pass MIDI timing data between applications. This is likely already on your system, but if it's not you can download it free from www.opcode.com.
Your computer must support Digidesign's DirectConnect plug-in. Digidesign lists the requirements for this as:
There are no known conflicts with either Pro Tools Free or the Reason demo, however, these applications are not officially supported.