If you can afford docking stations and/or external USB drives, the simplest is to make a backup image using a program like Macrium Reflect or Paragon Software or one of the other imaging programs.
If the machine becomes unbootable, or you just want to go back, you boot from CD and restore an image from the external drive. Some will also restore over network if you have networked drives.
On my systems I installed a USB 3.0 card. It gives you 2 USB ports. I plug in a USB 3.0 docking station and just stick an internal drive into the dock. USB 3.0 is fast. I have WD Caviar Black 6 Gbs and 3 Gbs internal drives I use in the docks. By keeping a lot of data on these I can keep my system disks lean enough that I can clean 'em up a bit with CCleaner, defrag, then have greater than 70% free space. I back up that image. Even from a USB 2.0 drive you can restore in less than an hour if you keep your system drives lean.
I haven't had to restore from USB 3.0 yet. I have to make a WinPE boot with the USB 3.0 driver to get the speed. (But I have USB 2.0 docks in case of emergency.)
For the additional storage it depends how you want to set up. If you want everything in the tower you may want to use some of those drive adapters that let you can plug drives in like sliding in a drawer. I don't mind having docks all over my desk so I went with docking stations. Or you can do NAT networked externals etc..
But the simplest with no messing around is, back up the image to USB 2.0 external drive. Make a boot CD. You can try a new OS. I put Windows Seven Beta on my Vista machine, tried Windows Seven 64 bit. Didn't like it. Put the Vista64 that came with the machine back on. Using the images and Macrium Reflect.
The main thing is make sure the restore program on the boot CD can see your HD and the external drive when you boot it. If you have a Raid driver or some other unusual hardware you can get an unpleasant surprise when you go to restore.
They are easy to use once you've done one. If you have an expendable machine or know someone with a guinea pig machine, you could do a backup and restore just to see how it goes. When your machine won't boot the mood is usually one that hampers thinking. You don't want to figure it out then. If you've done a run through, then you have confidence you can fix it.
Also if you can afford more than one external or use the approach that allows you to remove internal drives, it's a good idea to keep a backup image not connected to the machine. If you get a virus that spreads across your Lan you can disinfect, then hook up the external that wasn't connected to get a safe restore.
For free backup programs for imaging you can try:http://www.thefreeco...backupandimage.shtml