i have a HDD which has windows 7 installed, a few months ago i purchased a SSD and i installed windows 7 onto this, i still have my HDD plugged in too which still has windows 7 on it, i still keep my windows 7 on my HDD as i still have a few games on there i need to complete, when i've finished with the games i'm going to wipe clean my HDD and just use it as storage, windows 7 on a SSD is so fast, you wont regret it!
If you wanted to copy your windows 7 from your HDD to your SSD (to keep your programs etc) there's a few programs that will clone it for you, not sure what they are called, as for me, i wanted to start fresh so i installed windows 7 onto the SSD