You can look at this link: http://www.objectiflune.com/OL/en-CA/Services/Support/KBArticle.aspx?KB=1465
It says version 6, but it's the same for version 7. Also, version 7 will also work on Windows Vista, 2008 and 7.
Keep in mind that the actual PC that you'll need heavily depends on the workflow you're going to do. If you're just using simple text data with a simple form with just variable text, you'll be able to process it even if you have a weak computer. On the other hand, if your workflow involves thousands of large data files with forms with large images, it's going to take a lot of time and resources, even on a very powerful machine.
So take the requirements as a reference, but keep in mind that optimizing your workflow is what will really make a difference.