1) The Clawhammer will 'overtime' replace the Athlon XP.
2) The Clawhammer is completely different to the Athlon XP so there is no upgrade without upgrading the mobo as well.
Unless you have a specific need for 64bit or must have the latest thing I would suggest you buy an Athlon XP and start enjoying the system now (or once the T-bred arrives). Then you can upgrade to a Barton core or Barton-s as you need more power. Just make sure you get a decent mobo which handles 166FSB and PC2700 memory
<font color=blue> The Opteration was a success... I'm now a full-wit</font color=blue>