When new MB and cpu get to me will I be able to just plug hard drive in and boot up? Hope I don't have to reformat
Oh...you're an upgrade virgin, huh?
Gamester is absolutely right. Best route is a fresh copy of Windows. If you just swap in the HDD, you'll be lucky if you can get into Windows. And in the event that you do, you'll have so many problems to fix that it's just not worth it. You could try to install over the top of the existing Windows, but I wouldn't put money on your other data remaining intact.
If you don't have a means of backing up your entire HDD, I would suggest getting another small HDD somewhere (60-100GB). Smaller drives can be found very cheap on eBay, and many online retailers. Use the small drive in the new system to install Windows on. Then you can plug in your current one as a secondary drive. That's the happiest medium between cheap/safe/easy.
Obviously the cleaner way is to back up any "extra files" (music, ROMs, etc.) from the current drive, then do your reformat/installation, and copy whatever you need onto the new installation.
Another possibility is to create a new partition on your current drive and move your extra files over there, leaving only Windows on the first one. When you reinstall Windows, you only need to format the one partition, leaving your data intact. However, that's only an option if you have more free space than your extra files will take up (need free space to create a partition).