I live in Prince Georges County, MD and would like to know what zone (for gardening) I'm in?

Yes, it still matters, unless your garden has some kind of shelter that makes it equivalent or almost equivalent to a greenhouse. You do find plants that are borderline for your zone, are really better suited to a warmer zone, but can do fine in your zone if they are in a sheltered sunny spot such as the south wall of a house, whereas they might not survive 200 feet away on a windy hilltop. If you have a raised bed garden, those do tend to warm up a little sooner in spring than gardens which are not raised.

You should consult a garden store

http://planthardiness.ars.usda.gov/PHZMWeb/

Here's a chart, it will go by your zip code.

No