the easy way i found to do this was being i had a extra external around (western digital 250GB portable) i just cracked it open not literally but uno i got it open an i backed up my ps3 up to my other external it was fast cause i did the as soon as i got the ps3..but yea after you've backed up unplug remove hard drive from bay insert new one.
IMPORTANT!!!! be sure to have the ps3 software file on your external too in a folder under PS3. An sync ur trophies. lol.. plug up external an start ps3 an it will say sum stuff simply press start an select at the same time an it will load the ps3 software on the new hard drive. after it finshes simply go to restore from backup so u can get all your game saves an stuff.. i recommend this for all top do.. u will love it