I went to Task Manager. I have 644 MB used, but 19 MB is the market, 13 MB is settings, and 11 MB is mail. So I guess that leaves 601 MB taken up by the OS, Sense, and background stuff.
Edit: I'm not sure exactly what you're referring to.
I just went to "Running Applications" and it shows 327 MB of RAM used but that includes lots of stuff running in the background (Settings, Voicemail, Calendar, Google Services, Lookout, Facebook, Maps, SwiftKey, etc). HTC Sense is taking up 2.3 MB