What are the main differences between the old Powermac G5 dual 1.8 ghz (rev A model) and the new PowerMac G5 dual 1.8 ghz (rev b model)? I'm curious because it looks to me like the old dual 1.8 has a bigger hard drive and memory. Is there anything I should be concerned about when buying the old dual 1.8 versus the new one? Has anything been tweaked or updated? Please let me know which one I should buy...... Is the G5 chip the same or is it different and are their any heat or fan issues with the old model?