You don't have to reinstall OS, or any appz that do not depend on specific hardware. For example, I went from Athlon XP, to A64 x2, to Conroe, to Yorkfield, all with the same exact copy of Windows XP SP2. Each one had a different motherboard. Now, I had to make small tweaks to make sure the software all run with the hardware after each upgrade, but for the most part, they just transitioned nicely.
That said, I did recently install Vista 64bit on my Yorkfield, and I am very happy that I did a clean install. Things feel smoother, and you dont' get mysterious errors. So if you are doing this for the long run, reinstall OS would be recommended.