I agree with rugger, do a clean install. Not having to deal with all the old drivers and settings in Windows will be a blessing in the long run. As a general rule, I always format when making a "major" hardware change, as it just makes life easier.
i just switched from an AMD XP 2200+ to a AMD X2 4200+ and you can thank mr. gates for the most picky OS in the world cause anything more than basic hardware changes will at best require you to re-activate windows and at worse, (like my case) just cause an endless loop of BSOD's and you have to reinstall everything. like rugger said, back it all up to a spare HDD or a DVD and then reinstall everything, dont learn the hard way like me and have to pull your old HDD and slave it to someone else's to get your stuff off of it