i would compare the 1.2 tualatin 512kb processor to maybe a 1.8 to 2ghz P4 northwood
they were pretty powerfull when compared to the P4
but this isn't taking into account the performance hits you might take from the memory/chipset subsystems
also the comparison would be highly dependent on the kind of apps/games you are running
seti and anything that relies heavily on the alu and fpu should benefit from the lower ipc of the tualatin
where as media encoding/decoding, anything that is heavily optimized for sse2 and anything that likes HT will benefity from the P4 and it's whole "design philiosophy"
the turning point for games didn't really go to the P4 till the northwood core broke 2ghz imho (when compared to the tualatin and the k7)
hope this helps