A fast 50mm will likely be handy in low light situations if the focal length suits. For interiors I'm guessing something wider than a 50mm on a 300D is going to be needed...do you have a tripod you can put your 18-50mm on? If not and you can't use a tripod a lens with IS may more useful than a fast lens or a new camera body.
With a tripod you won't need high ISOs so unless you need twice as many pixels I wouldn't worry about upgrading the camera body just yet.