I do not think, that they have 64bit intructions. But the core duo has vanderpool support locked. In summer will be yonahs available with 64bit support, not yet.
But what do you expect from 64bit? Double speed?
No, nothing you will feel.
I have runnning for months my nforce3 amd64 3200 PC with a pure 64bit linux system. UT2004 64bit also tried. The situation is, that all drivers and apps have to be written/compiled with 64bit instructions and the system have to support it. In linux the most is done since years...
But windows users are living behind the moon with the 64bit situation, until Vista is coming in the end of 2006 or maybe next year, who knows. Then intel is driving with microsoft on one line.