Many people have forgotten if their home movies are 'regular-8mm' (also called 'standard 8mm' or 'double 8mm') film or super 8mm film. One way to determine which you have is actually quite simple: super-8mm film is released in 1965 so if you are certain your 8mm. films were shot before 1965, you have 'regular 8'. This doesn't mean that films shot after 1965 are automatically in the super-8 format! Regular-8mm is still made, even today.
A standard/regular/double 8mm projector will not play super 8mm movies. A super 8 projector will not play standard/double/regular 8mm movie films. In fact if you mix them up there is a good chance that your precious films will be destroyed.
Some people have both regular-8 films and super-8 and if you want to project them on one, single projector then you'll have to use a DUAL 8 projector and you can find plenty of them on eBay, look at the listings on the right side of this page.
What's it worth? Take a look at this list of: sold Dual Projector for a value indication.