If your talking about 300 routes then that is no issue at all. I have nearly 3000 routes in my area 0. If your talking about 300 routers then you'll want to use areas for sure. I have about 50 routers in one area.
I haven't yet began using more then just an area 0 and ospf has been absolutely solid and fast for well over a year. I know that I'll be quickly approaching the limits and I am beginning to plan for some route summarization and area creation.