via The Register -

Visit Apple's Advanced Computing Group web page at http://developer.apple.com/hardware/ve/acgresearch.html and scroll to the bottom. You will find a link to (an admittedly dated as it lists a 500 Mhz G4 as the fastest available) white paper from NASA evaluating the G4 for scientific computation against Cray, Alpha, MIPS, and P3 based systems in terms of flops per dollar and concluded that the G4 offers "bang for the buck" advantages in factors of between 5 and 8 over Alpha and P3 systems.

Here's that link: Research from the Advanced Computation Group

An Evaluation of PowerMac G4 Systems for FORTRAN-based Scientific Computing with Application to Computational Fluid Dynamics Simulation

by Craig A. Hunter, NASA Langley Research Center

http://math.wm.edu/~cahunter/NASA_G4_Study.pdf

Very interesting reading. Shame I've already written my exam questions ;-)