Fully agree with those who advised against RAID0 for you. Besides, these days RAID0 does not give you a BIG speed improvement. However, an SSD WILL be noticeably faster than any storage system based on rotating mechanical drives. You would disconnect your current HDD, then install the SSD, then re-install your OS on it and use it as your boot drive called C:. After that you re-connect your HDD.
Now want to tell Windows that you want to change the location of many of its default Folders like My Programs, My Documents, My Photos, etc. and put them on the D: drive, too. That way you won't fill up the SSD (fast, but limited capacity) with stuff you don't really need FAST access to.
Now you still have a bit of a problem. All the application software you've already installed on your old drive will need to be re-installed. You see, "install" also includes placing data in the Windows Registry on where the app software is located. A new installation of your OS on the SSD will have no clue that you have app software sitting on the HDD, so you must Install to get the Registry clued in. And in doing so, for the majority of apps you will want it to install NOT to your C: drive (as is usually the default), but to the D: drive (that is, your HDD that now has a new name).
THEN for each of the re-installed apps, you should go into them and change (if necessary) the locations of THEIR default folders for files you create. (Most of the time they will try to put the files on C:, although some may put them in My Documents which you already have told Windows to place on the D: drive, right?)