Turion x2s are able to compete with core 2 duos, but the problem is that they ahvent seen the price cuts AMDs desktop processors have had. So core 2 duo(merom) is always better value for money for laptops.
As for 64bit OSs, the 64bit version of Vista is a huge improvement from what ive read. However, the migration to 64bit processors wont happen for a while due to so many processors out there that dont have amd64 or em64t, including processors as recent as the yonah core.