In the first test that I've seen involving fluid dynamics simulations, xlr8yourmac has published a benchmark done by a knowledgeable user who has used the top of the line Fortran compilers and compared single-processor G4, G5, Itanium, Xeon, and Cray X1 processors.

The G5 starts out a little weak (beating the Xeon, but losing to the Itaniums), but then hits its stride at simulations with "real-world" size data and tops out at 500MB by beating the Itaniums (and everything else except the Cray) handedly.