i would hang on to the body you have until you build up a decent lens collection and really learn all of the functions of your camera unless you want a second body. the 30d is a semi-pro camera and while it shares a lot of the same features of the 400d, its layout is supposed to be such that it is easier to access a lot of those key features. but, there again, to me, it seems more logical to stick with what you have until you understand all of your settings and are able to set up your camera for your shot quickly.

as time goes by and technology progresses, you're always going to be able to find a deal on any camera you're looking at now.