I apologize for the delayed response here!
There really is no simple equation to figure out how much memory you're using when you're on a shared server. A very basic way to determine that is 15,000 total hits in a day can = 300 MB. However that is very basic and not 100% accurate; Web scripts are so dynamic, we cannot determine if you need to upgrade by total traffic alone.
What you can do is contact our support team (www.dhurl.org/dhsupport) with the domain you're inquiring about. We can check the logs on your server to help determine if you need a VPS, but we cannot determine the exact RAM usage for your site.
We also offer a one-week free trial for the VPS, so you're welcome to try it out with your site and see how it fits you.