I sequence, record, and mix on cubase SX and before that cubase vst 5.1 and I believe that 400MHZ is not sufficient for serious recording. You said you want to use the latest cubase. Well, that will be SX or SL. In this case, it will not cut it. But if you used older programs, this might do it. I am not very familiar with MACs and so reality check will always be good.