OS X :: Backup Data Using Time Machine / Saving Old Backups?

So I just successfully migrated all of my data and settings from my old iMac to my new 27 inch iMac using the Time machine backup on my Western Digital 1 TB external hard drive. Aside from the Time machine backup folder I also dragged a few other things over to the external hard drive because there was so much extra space on it. The drive is not partitioned.

So I'd like to start using Time machine with this new 27 inch iMac but the empty space that's left on the external hard drive is barely large enough to cover what I now have on the internal hard drive on my new 27 inch iMac. If I were to remove the Time machine backup from my old iMac there would be plenty of space again unless starting Time Machine on this new 27 inch iMac will just copy right over that backup file? Everything on this new iMac, the data and settings and so forth, is exactly the same. So is the user account. However the previous Mac was running Leopard while this new iMac is running Snow Leopard.

So if it doesn't copy over the old backup it will just create a new backup file? And if it does create a new backup file is there any point to keeping the old backup file since I no longer have the old iMac and wouldn't be able to access it again? Obviously if I didn't have to keep it I could erase it which would create plenty of extra space for me to start using Time machine again but with this new iMac.