hi, the short answers are yes (need to format/sys the disc 1st) and it it all goes wrong then yes you put the old one back in and everything will be as before. Once the new disc is up and running ok you can always put it back in a slave drive and use as extra space/swap file/backups/games etc!!
Any probs post back as always someone here to help
Nobody's answered yet, so I'll start the ball rolling!
Do you really want to replace your existing HDD? The simplest thing is to install your new one as a second drive. You just install it in a spare bay, and then connect up a spare power lead and IDE lead to it (the IDE lead to your existing HDD probably has a spare plug on it, which is what you use -- if it doesn't, you will have to buy a new lead which does have two plugs on it).
You then have to set the jumper on the original drive to "Master", and the one on the new drive to "Slave". When you boot up the new drive should be recognised immediately, and all you have to do is format it.
But, to answer your original question, if you just swap drives and haven't changed your BIOS, when you swap your old drive back in it should start up as it used to.
Before I had XP and large drives I used to have 98SE installed on several drives for the same machine and swap them around regularly. Might not have been strictly within the meaning of the license but it was always for the same machine.
Just put your new drive in, format as normal, install and away you go. If you don't like it, take it out and replace - your CPU isn't going to know the difference.
If you are going to replace the old with the new you will need to go into the bios first to remove old and detect the new HD. you will then have to Fdisk/format the hd. You can then do clean install.
If you have problems you can re install old drive and detect in bios.
If everything goes OK you can connect the second HD making sure you have set jumpers to slave if it is on the same ribbon, go into bios and detect second HD. You can now del all from old drive and use for what ever.