I'm actually slightly surprised that the X5000 beats a 2.3GHz G5 in most nbench tests, because I remember the opposite being true when testing the X5000 against my 2GHz G5. However, I just reran the tests and indeed the X5000 wins against my 2GHz G5 in most tests, and that's despite my X5000 only using single channel memory.

However, you need to remember that the CPU core used in the X5000 is basically a G3. A highly clocked G3 with 2 cores, but still a G3. You shouldn't expect miracles from it.