I would suggest taking a trip to Palo Alto and visit Keeble ad Schuchat Photography. They have a very helpful staff there and every kind of camera. If you wanted to test the waters you could even rent one for a weekend and see what you think.
I currently have the 40D and really like it. The difference between the megapixels between the two are not going to be a big issues unless you are planning on making pictures the size of a wall or such. It basically comes down to the features between the 40D and the Xsi (which is what the 450D is in the US) . The 40D is also consider a semi-pro camera which is basically just a fill in between the Rebel line and the 5-D and 1-D Mark III line, which is the professional camera.
The other thing to keep in mind is that lenses can be used between the Rebel and 40D with out any problem as the CMOS chip (also know as the image sensor), which is like the of the film in old cameras, is smaller, i.e. not the same size as a 35mm negative (the size is 22.2 mm), in the lower end cameras and the professional cameras are what is called full frame which means the image sensor is actually the same as a 35mm negative. But if you decided at some point down the road that you wanted to move up to a full frame camera many of the lenses you may have purchased will not work, the are not designed to product a full frame image.
So basically you need to look a 1. cost and 2. features. My suggestion is to take a trip to Palo Alto and talk to one of the people at KSP and try them out. They will show you both models and you can get the feel and understand the differences.
Hoped this help and didn't confuse things more.
Paul
