Q: "Is there a way to visually see the difference between the old PCI cards and the new ones for the Mac G5?"

A: We'll assume that you're referring to 5 V (see WFTD, Voltage) versus 3.3 V cards that were used in older PCI Macs. The Power Mac G5 computer is designed to the PCI-X 2.1 specification, which requires 3.3 V signaling. There is a difference in the new PCI slots for the G5 computer, which will help you differentiate between the two standards. The older 5 V cards are keyed differently from 3.3 V cards and will not fit into the computer's PCI slot. The new PCI slots (from front to back) have a short crevice followed by a long crevice while the old PCI slots have a long crevice followed by a short crevice. One close look at your machine and a card you may be holding in your hand will tell you if it can be used.