I have to strongly disagree with this and say to new buyers to take the above with a huge bag of salt. I've used a 400D, 450D, 40D and a 50D quite often and they all feel the same in build quality. I wouldn't say it felt any more plastic than the 40D/50D. It can take some immense shots (as seen on this forum) and it'll do everything a new photographer will want it to do. I first used a 400D about 2 years ago, and I bought my own 450D about a month ago. I'm still learning and although I'd like a 50D, I can't justify the money at the moment and that's why I went with the 450D. It is still doing everything I need it do at the moment, and I wouldn't say that I'm a slow learner.
There's so many different add-ons you can buy for it. Flash guns, lenses, filters, off-camera flash equiptment and so on -- if you feel you've out grown the 450D there's still tons and tons you can learn without needing more mega pixels or a faster burst. If you haven't got vast SLR experience, the 450D will keep you amused for several months unless you use it a lot more than the average user would.
I'd pick the 450D over the 40D all day long, simply because I wanted a new camera. If I was prepared to buy
second hand I probably would have bought a 40D admittedly, but I didn't want to because it's an old model now so I feel the 450D should be compared to the 50D. In that case, the 50D is £400 more than the 450D, and it doesn't come with the 18-55mm IS kit lens (which a new starter should probably invest in seeing as you won't get better for £40-£50).