Not 100% certain as I'm not a Canon fan - but it's my understanding that all three cameras take the same lenses. It'd be the Canon 5D that takes the lenses designed for "full frame" sensors.
If I was buying Canon, however, my preference would be for the EF-S 17-55mm f2.8 IS USM. I'd try and buy it second-hand, refurb, off a friend disillusioned with photography, or brave eBay to keep the price down; and I'd happily make any concession with regards to camera bodies to get it, even if that meant a 400D or, perhaps more interestingly, a second-hand 20D/30D.
I have no doubt that a Canon 350D with a fantastic pro lens would wipe the floor with a Canon 40D with a standard kit lens when it came to measurable image quality.
Of course, this isn't what the photographic press want to peddle these days. It's camera bodies, after all, that sell magazines, rather than lens reviews. But if lenses became all important, people would start wanting to know what happened to all those old lenses that can be picked up for nothing (but don't autofocus or stop down automatically)...
And *then*, $2/£1 bargains like this would become a thing of the past:
... once the public found out they were effortlessly capable of stupidly sharp photos like this:
Camera companies want you to believe that everything that's come before isn't as good but that's insanity. The twentieth century was documented with mechanical cameras and photographers who thought aperture priority or pattern metering were fanciful science fiction.
This week, I've been reading about a well-known British portrait photographer called Jane Bown:
http://lifeandhealth.guardian.co.uk...2197373,00.html
For the past forty years, she's used a lovely camera called an Olympus OM-1 (second-hand price is about $50 these days). Most of her pictures are taken with an Olympus 50mm f1.8 which go for $15-20. Her camera offers full manual mode, a range of shutter speeds and apertures, but she likes 1/60s and f2.8 - and finds the spot in the room that'll allow her to use those settings.
Now, let's say you wanted the convenience of digital. All you'd need is a camera that'd let you mount the same lens or something similar (and ther's plenty of them) - and you'd be, kit-wise, on equal footing with one of the greatest portrait photographers alive.
Makes you think, doesn't it? At the end of the day, if you truly know your photography, there isn't a single feature in a modern camera that you actually need. And, once you acknowledge that, you might as well buy anything that tickles your fancy - it doesn't matter! But knowing your photography is a different thing, a more difficult thing, from making a consumer purchasing decision - so that's why the press makes the whole discussion about kit.
My advice? Think less about the kit, and think more about the kind of images you want to make. It's easier in the long-run, and you'll enjoy photography more.