No one can completely answer your question without knowing details about your install, such as what plugins you are running and the memory requirements of each plugin.
Wordpress does not scale well without some sort of Caching plugin. W3 Total Cache and WP Super Cache are both great plugins that will let your site scale much beyond a default Wordpress install. These plugins essentially minimize the dynamic PHP compilation and SQL hits and instead serve HTML files.
It then becomes a calculation much like any other busy web server. How many images are coming off of your server? Is output gzipped? Are you planning for future growth? Many factors go into how fast the website will serve up and how much RAM you will need.
That being said, at your current size I think I would go with at least 1GB of RAM, assuming you use a caching plugin and have minimal graphics serving off of your server. However there are big caveats above.