Let me get a grip of what your asking..
Are you saying that you had a harddrive in your mac which you have now replaced with a larger one..
And you would like to grab all the information from that drive for use with the new drive?
There are a couple of ways to do this..
Your easiest option would have been while the drive was in the mac as the primary drive to use a 10.5 feature called "time machine" to create a backup of your files (everything apart from the operating system) to an external drive and then to use this backup to restore your computer to its original state after reinstallation..
However i am guessing this is now not an option as you have made a transition.. your other option would be to manually reinstall your applications and copy and paste your files over,
Some people also talk about using an application to clone your drive, but depending on how important your stuff is
Hope that was some vague help..
Gimmie a shout back!
BTW what kinda mac do you have?