I find it unusual that the G4 is faster than the Intel Core. The G4 architecture is ancient compared to the Core architecture and I would expect that Intel would have reduced the latency of most instructions (eg. add, subtract, etc.) to one or two cycles. Since the Core is also clocked way higher, along with faster RAM and that, it should have a significant performance increase over the G4.

When comparing single core to dual core processors, keep in mind that dual core processors usually have newer architectures than single core processors and that alone should make up for the 1 or so GHz difference. For example, the Core 2 has dual FPUs, while the P4 doesn't , so it would completely own the P4 even at a lower speed (to a certain point, of course).