I think it was Spring Hill Nursery's website that I was looking at that said our house is going to be in, but then I looked here and it said zone 8. How do I find out for certain?

As people start to use the USDA's new Plant Hardiness Zone map you will see some changes, but not eveyone is there yet. The USDA has a website with the new PHZ maps or you can look at this website http://www.growit.com/ZONES/ to see where you are.
Keep in mind that the Plant Hardiness Zones are simply a guide to tell which plants would survive an average winter in your area and nothing more.

I believe the absolute best way to tell what zone you are is is to ask a number of gardeners that live near you. I am in the NC mountains, West Asheville to be exact, and I say zone 6ish when we are lucky as the weather here varies so much due to altitude. It changes mile by mile as the terraine goes up and down. Locally, our 50% last frost date is May 1 and our 90% date is May 15.