I would think you are better to upgrade to the AMD64 since there is no point in upgrading from a 2500XP to a 3200XP. I mean you could easily overclock your 2500XP to the 3200XP's speeds so theres no point in wasting money on it. 2500XP Barton cores are known to be very good overclockers as depending on if you are very lucky, they can achieve speeds greater than the 3200XP. Of course your motherboard & memory are also important too.
Upgrading to the AMD64 will gain you huge performance benefits especially if you are into games for example. Only problem is & unless you have the money for it, you would have to buy a new motherboard and perhaps more RAM to achieve its full potential.