Don't forget that the fastest Athlon64 is over 1Ghz slower then the P4 and that P4 has dual channel to it's side.
And trust me, the Athlon 64 will do a bit better when run in 64-bit OS, but then again, nobody needs 64 bit atm.
If I were to buy a 64 bit cpu, it'd be the opteron 146, but I'd wait with that untill prices come down a little.
My guess is that the K8 was never really meant to take over the XP, but more just the server market, where it seems to do pretty well against the Xeon's and Itaniums, due to price/performance ratio again.