One way you can go about saving your old files, is to run the xp disc and partition some space from your main hard drive, or use partition magic to partition your existing 'old drive'. Move your files from the old drive into a partition, take your time to back them on disc for sure(data dvd using nero works too) and I think there is a way to find your CD key through windows registry?
Aside from that, I can't think of anything to retrieve a lost CD-Key, most people will tell you to buy another OEM disc which is what, roughly 100?
In the future it'd be wise to invest in a 320 external drive.But the smart or practical thing to do with any large hardware upgrade(cpu or a mobo) is to reinstall windows.there are a decent amount of bios differences and drivers from your old mobo on your hard drive that it'd be a pain to go through again and again.