Actually the 8th SPE is disabled completely to increase yields in manufacturing. One of the remaining 7 is running background tasks.
Also, the number of MHz they're running at is irrelevant when comparing the console CPUs to normal desktop CPUs. They're completely different beasts. In much the same way a 2GHz Pentium 4 would have its ass handed to it by a 2GHz Core 2, the PPE on the Cell chip would get royally spanked by any moderately recent x86 processor. The fact that is runs at 3GHz+ (it does) doesn't mean much for its actual performance.
Also, the number of MHz they're running at is irrelevant when comparing the console CPUs to normal desktop CPUs. They're completely different beasts. In much the same way a 2GHz Pentium 4 would have its ass handed to it by a 2GHz Core 2, the PPE on the Cell chip would get royally spanked by any moderately recent x86 processor. The fact that is runs at 3GHz+ (it does) doesn't mean much for its actual performance.