Since I own one my opinion may be a little influenced, but you'll get the same with nikon and canon users., so:
First, all entry-level cameras deliver practically the same amount of image quality, as long as their are 10 MP cameras, 6 MP cameras aren't good enough, why? because some times you need to crop, specially with the kit lens from any manufacturers, because they don't have enough tele-zoom.
So this is reduced to the "extra" features all DSLR have.
Canon XTi: In it's favor it has DOFP button (If its important is up to you, for me it's useless in their small viewfinders), the pop-up flash go higher and slightly better control noise in high ISO, this is an important factor, however, I find that turning off the Noise reduction of the A200 and then applying noise reduction in the computers is way better anyway for both cameras (it doesn't change the fact that is slightly better here). In A200 favor you have a more richer package of features, spot-metering, Super Steady Shot, a slightly, and far better, wider and longer (since canon crop factor is x1.6) kit lens, DRO, a slightly larger screen, 3200 ISO and an accurate battery life indicator, slightly bigger viewfinder, better build, ergonomics and battery life.
Canon XS: It has the same advantages the XTi had, plus Live View, however, unlike sony implementation, it's 100% accurate but slow (since it's contrast based live view), for me, unless you're shooting in a studio, it's way to slow, around 5 seconds to get focus, however, it may be useful for you usage, so think this carefully. The A200 advantages remain the same, with an additional more sophisticated 9-points AF.
Nikon D40x: In it's favor it has slightly better high-ISO performance and the flash pop-up higher, but the A200 clearly is a more featured camera, far more sophisticated 9-points AF, larger screen, accurate battery life, better battery life, Super Steady Shot, better ergonomics and build quality, DRO, longer kit lens (however, the nikon kit lens is a far better performer) and spot-metering.
An extra feature of the A200, its supposed to shot 3 fps, but reviewers said it was 2.8, however you can boost this with the right CF card, up to 3.4 fps, I dunno about the others.