Your CPUs score 442, 806, and 869 respectively on Passmark's list of CPU benchmarks. When running on a single core, your dual-core CPUs should perform roughly equivalent to a CPU with half their score. The single-core scores then are about 442, 403, 434 (i.e., all about the same, give or take a bit for clock/bus speed and cache size).
I assume pystone doesn't take advantage of multiprocessing, and your results back this up.
So, to answer your question, all of your processor cores are approximately the same performance class, so they all do approximately the same on the pystone test. The dual-core systems should be able to run two concurrent pystone's without much performance penalty though!
Here are a few other benchmarks from machines that I have handy (python 2.5 or 2.6):
Pystone Passmark
Dual PowerMac G4 @ 1GHz: 18587 n/a (probably about 300)
Pentium 4 @ 2.53GHz: 37313 327
Core2 Duo T7200 @ 2GHz: 51245 1129