It's always best to do a fresh install, that way windows doesn't try to put in any random drivers and mess things up.
If you were going from say a p5b to a p5b deluxe, you would not need to re-install because all you are doing is adding some features, like wifi to what windows would think is the same thing. But changing from a 965 chipset to a 680i will require a full format and reload.
I would personally always do a fresh install, just because everything works nice and fast after one, so your new, faster parts appear to be even better