Hi Ashley,
I was in the same position as you a couple of months ago and I couldn't decide between the Canon 40D and 450D. Long story short, I finally decided to get the 450D mainly because the 40D was too bulky and heavy for my liking.
As said above the 40D is a better camera overall but the 450D is a great performer. I have been using mine for astrophotography and I have had absolutely no problems with it. From my experience with it, even at a higher ISO setting it doesn't produce much noise. I am planning to get it modded soon and use it souly just for astrophotography.
No matter which camera you decide to get, you wont be disappointed. Hope this will help.