Kouroshs123 :
What about my windows product key, wouldn't Microsoft detect that it is installed on two drives?
No, because you're going to:
A. Not use the HDD one anymore
B. Remove it from the drive.
There is only one application I've seen that will move
only the OS to a different drive. And it's not free.
http://www.paragon-software.com/technologies/components/migrate-OS-to-SSD/
Depending on the size of your proposed SSD, it is better to have it all one one drive, except for games.
For instance, a 120GB can hold the OS and a BUNCH of applications.
Personally, I
always recommend just a clean install on the new drive.