I followed this guide;
http://vgstrategies.about.com/od/ps3cheatsandcodes/a/PS3HDDDriveSwap.htmI bought this HDD;
http://www.amazon.co.uk/gp/product/B0031M9JCEPhysically replacing it was easy as piss. The guide explains it. Just unscrew the old one and slot the new one in.
What the guide glances over is software side of it. I backed up my old HDD using the PS3's backup utility before changing it. But I later found it this was utterly poinless for me as all that transfers over is game saves. I have Plus so the cloud saving takes care of that. All your PSN content; games, themes, and avatars, will not transfer to the new HDD when you try to restore it. But if you don't have Plus the back up is necessary to keep our saves. Make sure you sync your trophies before changing the HDD, because those aren't included in the back up.
As for restoring the backup once the HDD is switched, you will need a USB storage device with the latest firmware on it. Create the file directory "PS3 > UPDATE", download the firmware from your region's Playstation site and put it in the UPDATE folder. When you first start up your PS3 it will say the HDD is incorrect and the system won't start. Turn it off and on again and it will ask for the firmware. Stick the USB stick in the PS3 and continue. It will format the HDD and install the firmware. Then you can restore your backup.
That's how it all worked for me.