Just though I would share my experience of using ARP2600 on a below spec machine. I am actually using a Pismo G3 Powerbook! This has been upgraded to a 550MHz G4 by Daystar and was surprised that things run smoothly - although Arturia do suggest a 1GHz minimum for Mac. Thought I would push things a bit and run Logic with ARP plug-in running a complex sequence and then layer in via Rewire a heavy Reason track (running 14 NNXT's). IT WORKS! The CPU meter in Logic only gets to about 3/4 and it syncs great.
Just thought I would share this with those perhaps put off a bit by the 'aggressive' system requirements.