Some people will tell you to clone the HDD, I am NOT one of them.
As fresh install of the OS is always best to get the most out of the SSD. Windows 7 has special features for SSD that won't be there is you clone. And cloning takes almost as long as a fresh install (IMO).
As far as the product key, you should be okay. Hard drives (or your case upgrading to a SSD) are not as big a deal as a CPU and motherboard upgrade. Use your disk, use your key.
Here are some tips & trick for SSD users:The SSD Review - SSD Optimization Guide...OCZ Blog - SSD Tips & Tweaks
You don' thave to do them all, or any, They are just tips.
If the HDD had any version of Windows on it:
1.) Once you install the SSD, unplug the HDD before you install the OS to the SSD. After install you can plug the HDD back in.
2.) Move the Libraires (i.e. My Documents, Downloads, Music, Pictures, Videos, and any Gaming Data) to the root of the drive (e.g. D:User/My Docs to just D:My Docs, so one and so forth). This can be done with drag and drop, or right click on the folder, and change the location as noted.
3.) Move the new Libraries to the storage drive (e.g. C:User/My Docs to D:My Docs).
4.) Move all you User data to the new drive (e.g. D:User/Contacs to C:User/Contacts). This will "save" some of your user specific data (not settings). The folders you should move to the OS drive are: Contacts, Favorites, Links, Searches, and Saved Games. Do not move the AppData folder (which is hidden anyways), this will be updated with all the programs you install (i.e. Adobe, iTunes, Office).
Then you can delete all the Windows stuff off of it, all the stuff not part of your Libraries (i.e. "Windows" and "Program Files" folders)P.S. Think I should write a how-to sticky? Think I can?