I can't quite answer the above question - understanding Intel's logic is ... well - a feat beyond me. Marketing ... bleh.
Anyway, the point MikeC
is very true - a 2.4 GHz Celeron will perform (roughly) like a P4 1.4-1.6 GHz - yes, it *IS* that bad. P4 is hungry not only for Memory bandwidth, but also cache - and the Celery's mesely 128 kb L2 are just starving it.
Get the P4 2.0 GHz (just make sure it's the Northwood - 0.13 micron, not Wilamette - 0.18 micron, as Northwood has 512 KB L2 cache, over Wilamette's 256 KB). It's a pretty nifty CPU - most of my work-lab PC's have it, and it's a good soldier