Install the SSD first . . . no point installing Windows to the old drive, then doing it again
to the SSD.
Edit: By definition, if you install the OS to the existing drive, the SSD will not be your boot drive. I've never tried upgrading the OS to another drive; not sure if that's even possible?
In any case, the only reason not to do a clean install is if you have programs that you can't/don't want to reinstall. If that's the case, I'd use clonezilla or similar to clone the existing drive to the SSD, then run the Win 8 upgrade.
Otherwise, you can leave the existing drive untouced, add the SSD, and install Windows 8 there, making sure to boot off of that drive. You can then access the existing drive as a data drive, and get any files/data off there that you'll need.