CPU are more efficient now and can do more in 1 clock cycle. this is due to many things mostly due to better use of cache and better branch predictions. That celeron can probably do 2x as much per clock cycle than the pentium 4 so its really not much of a surprise that it beats the 2.4ghz pentium 4 even with its slower 1.6ghz clock.
What do you think would be the largest contributing factor to the newer 1.6 cpu beating out the older 2.4 one?
They dont list the actual microarchitecture used on those specs, it's just some general compatibility stuff... The microarch affects a lot more internall operations that aren't visible on the surface...
Some simple benchmarks like the windows experience index or PCmark give a rough estimate of the cpu performance but isn't 100% accurate. If you are looking to buy a now processor, you can check out a review of it and see how it performs.
CPU performance depends a lot on software so know what you run and then decide what kind of performance you want and need.
^^yep, you'd better check some performance reviews on sites like Toms hardware, Anandtech, TechPowerUp, HardwareCanucks etc...
And some software/programs prefer more cores and some prefer higher single core performance, whether it's due to higher clocks or more efficient microarch is pretty meaningless, only the end result matters.