2.4 is a Northwood, running 400 (or 100) FSB
the 2.4b is also a Northwood, running 533 (or 133) FSB.
Any p4 with a 512K cache is a Northy.
2.4b is more desirable for OCing, as it has a lower multiplier. Both chips might theoretically have the same ceiling, but because the 2.4b has a lower multiplier (and therefore higher FSB), you'll get better performance with the increased bandwidth.