I just started designing a large game site and would like to know what if using multiple databases & servers is a good idea since a site getting a lot of quieries to a database might over stress the server (correct me if i"m wrong)

The website will include

User management
video uploading, steaming and pasting
forums
blogs
articles
image galleries
chats

I wait for your helpful suggestions

Multiple databases is necessary, but the server is not.If the shared server cannot meet your requirement,you can also choose a dedicated one.
You can optimize the query entries,that would be benefit for your site.
In my opinion,the entries should be less than 500 in one table.

You don't have to worry about your website. You just need one server but multiple database is necessary. You should use MYSQL as you database.