NASAis Langley Research Center has compared a dual 2.0GHz Power Mac G5 system to a dual 1.0GHz Xserve, a dual 1.25GHz Power Mac G4 (all three Macs used only one processor), a 2.0GHz Pentium 4 system and a 2.66GHz Pentium 4 system using its Jet3D fluid dynamics simulation application.
We were alerted to this comparison through an article at OSNews.com, which pointed us to a report posted on the Internet by Craig A. Hunter of the NASA Langley Research Center. The report includes highly detailed system specs and compile flags for the tests. The scalar version of the application is written in a mixture of FORTRAN F77 and F90, with the vector version written in FORTRAN and C. A G5-specific FORTRAN compiler does not yet exist, so the version of Jet3D running on the G5 is the exact same version run on the G4. Despite this, the results showed that the Power Mac G5 is one fast Mac. From the site:
This paper describes testing conducted by NASA Langley Research Center during an evaluation of a PowerMac G5 system for computational fluid dynamics (CFD) simulation. Preliminary evaluations are conducted in Part I of this two-part document. In this phase of testing, an existing version of the NASA Jet3D  code (developed and compiled on G4 systems) was run on a G5 system without any re-compilation or additional optimization. As G5-specific FORTRAN compiler tools have not yet been released, this is both appropriate and a reality. In Part II of this document, G5-specific testing will be conducted as revised compiler tools become available. In addition, testing will be expanded to include other NASA CFD codes.
The primary purpose of this test is to determine how G5 performance compares to G4 performance in CFD applications. Earlier work  showed that G4 systems performed well in vector computations but fared quite poorly in general scalar floating point computations. Since CFD simulations are heavily dependent on basic floating point performance, it is a critical area of evaluation. As a secondary part of this test, G4 and G5 benchmark results are compared to similar results obtained on Pentium 4 systems. Jet3D was compiled and optimized for the best performance on each platform, using a mix of available compiler tools on each platform. Thus, cross-platform results shown herein are not directly comparable in a true sense, but rather, reflect the reality of working in a multi-platform environment. Therefore, it is important to note that these tests are not academic or marketing benchmarks; rather, they are real-world tests conducted in a multi-platform engineering environment.
You can read the full study at Craig Coxis Web site.