SQL server space

I created an online highscore table using php and SQL for a flash game I developed. The game consists of 40 levels, and the table shows the top 20 scores of each level. Since I’ve never done anything like this before, could someone give me some sort of perspective as to when I should be concerned with the available space in the server? Is there a way to check how much space is remaining? I would assume it would make sense to not even bother saving a submitted score if it didn’t make the top 20, but is it that important to do so?