What you do, is you save all of your game saves to a memory card. then you transfer the gamesaves from the memory card to the new hard drive. then you recover your gamertag from xbox live. simple as that ;)
Theirs an easier way than that,request a transer kit from MS.com (it will be free)so you can connect your 20GB Hard drive to your 120GB hard drive and move ALL the data off your old hard drive.
hey no way i thought i would have to pay for that, even better again. hmm... i wonder if they would transfer my extended warranty i bought that'd be great