I'll point out that I'm not sure I trust the "trusted reviews" site on the issue of high ISO noise. The samples they posted are all crops of a well lit, and well exposed door. This doesn't show the noise problems that tend to occur in more poorly exposed shadow areas, and the natural graininess of the wood may hide a bit of the grain as well.
For more reviews look here:
Here's a couple which I think provide a more balanced view:
The CAmeralabs review provides a nice summary of the differences with the Canon 400D:
"The Canon EOS 400D / XTi is already the best-selling 10 Megapixel DSLR and arguably the E-400's biggest rival. Interestingly though, two of the aspects the 400D / XTi sells itself on – compact size and anti-dust – are both bettered by the Olympus. The 400D / XTi may also offer a wide variety of anti-dust options, but in our tests they were less effective the E-400. The Olympus body is also noticeably smaller than the 400D without compromising comfort. The E-400's kit lens is additionally superior in many optical respects, and unlike the Canon kit, features internal focusing and a lens hood.
So far it sounds like the E-400's winning hands down, but the Canon 400D / XTi delivers lower apparent noise, especially at higher sensitivities and fractionally out-resolves it in technical charts; the view through the viewfinder is also wider. So if you demand silky-smooth JPEGs out of the camera at high sensitivities and prefer a bigger grip to wrap your fingers around, go for the Canon. Crucially, the 400D / XTi is also working out cheaper from internet dealers."
All of the four-thirds lenses will autofocus. The biggest problem with the lineup is the lack of fast prime lenses for more challenging lighting conditions. You can only use older manual focus prime lenses with an adaptor. The Sigma 30mm f1.4 I mentioned earlier may be all you need to adress this.
That tradeoff may be worth your while. For the subjects you so far have expressed an interest in shooting you would do well with the Olympus. And it is about 20% lighter than the XT when loaded with a lens. And the second kit lens, the 40-150, also is very light at only about 220g, and with the 2X factor provides a 80-300mm equivalent zoom range.
Also keep an eye out for new models introduced next week. It is rumoured that Olympus might intoduce in-body stabilization, but I think that might be only in a larger higher priced model. There is a good chance however, that the E-400 replacement might get "live view" - the ability to use the LCD for composing shots. This would be very useful for your macro shots, as it allows you to fine tune the manual focus more easily (using the tripod is also recommended).
The Canon 400D in contrast would be more versatile in challenging lighting conditions. It would easily beat the Olympus in situations like shooting indoor sports, or concert and theatre type shots without flash. The combination of a more usable ISO 1600 and fast prime lenses will give a notable advantage there. Autofocus may be quicker as well.