You will definately see an improvement in Image Quality by using an L Lens on a 400D, when compared to the kit and most non-L lenses, you wont get the absolute best out of the lens, but you will get the best out of the camera.
You're neither Stanley Kubrick or NASA, so you're probably out of luck.
This is a very simplified explaination but it goes something like this:-but I can't see why it costs SOO much to make fast lenses.
I got L glass with my 350D, it made a huge difference. Then when I moved to a 5D body, it was nice and everything but the IQ didn't really change, I just got more options. I fully recommend better glass over bodies!