sorry to jump in here but are you saying you must run 64 bit version of XP to run new processors or do you say that solely on the fact of the ram limitation of 32 bit OS? And if it's due to something else, what is the problem?
Perhaps I should have said "to get the mosst out of the money they cost, use the 64 bit version". In theory, the 32 bit XP would go in with, as you say, a memory use limitation and programmes using only 32 bit wold run quite poorly.
Nothing in the CPU itself will block you from running XP on the system, but everything else needs to have XP drivers to function. Many new computers don't come with XP support.