To get to any other OS as an upgrade, you would need to start with El Capitan: https://support.apple.com/HT206886
Sierra and High Sierra won't install as upgrades over 10.6.8.
SSD will greatly speed up your Mac and its a good idea.
Given that High Sierra has just been released it might be better idea to wait 2 or 3 updates before installing it. Keep in mind that High Sierra has mandatory new APFS filesystem if you install it on SSD. New filesystem may have issues so waiting may be better choice...