When I added an SSD I did a fresh install on the SSD but left the previous install of win 7 on the HDD. It doesn't get in the way, I still use the HDD as a storage/programs drive. Even some of the previously installed games run fine from the HDD. Leaving it there also gives you a bootable drive in case you have a problem with the SSD.
Then how do you activate the Windows on SSD? Can you use the same key as in HDD even in this scenario? Also is there any issue when all existing drivers and registries are on the old HDD, not on SSD where the working OS resides?