This seems to be speculation at this point, but I have read in some reviews
of the new Celerons that intel may have increased the cache latency on these
processors to keep them from threatening their line of PIII coppermines
(like the old 300a's did the early PII 450s).  In these reviews, the authors
were noting how the overclocked new celerons underperformed the coppermines
by a large margin when comparing comparable clock speeds.  I don't know how
much cache latency could affect performance of SETI calculations, but an
experiment would be to turn off the L2 cache on both processors and see how
the numbers compare.  I'd be interested in hearing the results if you try
this.

John Sproule

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