From Oleksandr's comments:
From my limited experience of Geekbench, the odd choice of weightings
for the different components makes the result rather meaningless as a
practical performance metric. This is a general problem: any real
workload emphasizes particular aspects of performance more than
others, and unless you can define the workload very well, you will not
be able to make reliable inferences from benchmark scores. For
Mathematica this is a particularly serious problem, since everyone
uses it in a different way. The built-in benchmark is mostly
reasonable, but might not be fully representative of your usage.
As Erik Brown remarked, the built-in benchmark can be used as follows:
Needs["Benchmarking`"];
Benchmark[]
It has results for some computer systems built-in as well:
$BenchmarkSystems
3.07 GHz Core i7-950 (8 Cores) (Windows 7 Pro (64-bit) Desktop)
1.73 GHz Core i7-820QM (8 Cores) (Windows 7 Ultimate (64-bit) Laptop)
2.4 Ghz Core 2 Duo Mobile T8300 (2 Cores) (MacBook OS X Snow Leopard (64-bit) Laptop)
2.80 GHz Core 2 Duo Mobile T9600 (2 Cores) (Windows 7 Pro (64-bit) Laptop)
1.6 GHz Core 2 Duo Mobile L7500 (2 Cores) (Windows 7 Pro (32-bit) Laptop)
2.93 GHz Core i7-940 (8 Cores) (Linux Ubuntu (64-bit) Desktop)
3.00 GHz Core 2 Duo E8400 (2 Cores) (Linux Ubuntu (64-bit) Desktop)
2.60 GHz Core 2 Duo Mobile T7800 (2 Cores) (Windows XP Pro (32-bit) Laptop)
2.13 GHz Core 2 Duo E6400 (2 Cores) (Windows Vista (32-bit) Server)
2 x 2.66 GHz Dual Core Xeon 5150 (4 Cores) (MacPro OS X Snow Leopard (64-bit) Server)
2 x 2.26 GHz Quad Core Xeon E5520 (8 Cores) (Mac XServe OS X (64-bit) Server)
2 x 2.00 GHz G5 PowerPC (2 Cores) (Mac OS X (32-bit) Desktop)
3.06 GHz Core 2 Duo E8435 (2 Cores) (iMac OS X Snow Leopard (64-bit) Desktop)
2.67 GHz Core 2 Quad Q9450 (4 Cores) (Linux Debian (64-bit) Desktop)
2 x 2.80 GHz Opteron 254 (2 Cores) (Windows XP Pro (64-bit) Server)


Needs["Benchmarking\"]; Benchmark[]` Now if I can find a webpage that has a list of results for different machines... – Eric Brown Jul 07 '13 at 02:54{"MachineName" -> "6-node homogeneous cluster", "System" -> "Windows-x86-64", "BenchmarkName" -> "MathematicaMark9", "FullVersionNumber" -> "9.0.1", "Date" -> "July 7, 2013", "BenchmarkResult" -> 3.802, "TotalTime" -> 65.531}. – Rod Jul 07 '13 at 13:12