As I recall, the quad screen is just a screen, not a test, so it only tells you the percent chance of your baby having DownSyn. In any medical (or even educational) situation, "screening tools" are not the same as "tests." You can think of it as information to see if you should go on to do more "official" testing.
For DownSyn, amnio is a lot more accurate, because it's a test. But, it's a good idea to think about why you want the test results before you undergo amnio(e.g., do you just want to prepare? Or would you consider aborting?), because amnio does carry a risk of miscarriage, albeit small... so you may not want to risk it if it's just to gain information.
As for aborting if it's positive, no test is 100% accurate. My mom was told that my sister had DownSyn, but was one of those 1 in 100 where they were wrong. Thankfully, my mom hadn't aborted.
I know it's a lot to think about. Best of luck!