1. Yes you can use your licence key with either the 32 or 64 bit version of the same OS, so if you have W7 Home Premium the key will work on both. You need to use the same service pack version as well, if your original install is without SP1 then you will need a pre-SP1 DVD.
2. It is unlikely you will get a "free" disk from Samsung as they do not supply them in the first place, you will not get one from Microsoft unless you buy a licence to go with the disk. As you already have one there is no point.
Your issue is going to be the drivers and preloaded software. You can download the 64 bit drivers from Samsung, much of the software pre-installed will not be available on the site unless it is produced by Samsung.
You can download (legally) a copy of Windows 7 here, make sure you choose the correct language and the 64 bit edition from the list.