I know someone who got married at Newton White. They loved it. We were going to look at it, but it was too big for our needs, so we decided not to spend the money for it.
Since I'm not at home right now, I can't look at my wedding files. I know there are a couple more places, I'm just not sure they hold that many people.
Are you absolutely stuck on PG county? We had our reception in Columbia at Historic Oakland. It is reasonably priced. The only thing that would be bad (or good depending on your tastes) is that you would have to have a tent with that size group. I'll look through my emails and files when I'm back home on Monday night.