Hyp3rnova, they are all good cameras and will perform well for what you want.

Of the two you propose, I'd say the Olympus E-510 is the best.
The Canon 400D (or Rebel xti) has been surpassed in quality by a number of other brands, not just Olympus. But that hasn't stopped the majority of camera buyers from purchasing one... Some reasons: availability, lens options, upgradeability, the company's widespread fame...

If you look at the user reviews (the appreciation of the users/owners for their camera) on dpreview for the two cameras, you'll see that the Olympus E-510 gets a rating of 4.5 stars, whereas the Canon 400D only gets a 4.25... That should tell you something; but again, that doesn't say everything...

Why I would choose the Olympus: it's better built, better in the hand, it has double in-body Image Stabilization that works really well (so you don't need to buy expensive fast lenses with IS), the kit lenses are the best on the market, and the Olympus lenses are very good and compact, thanks to the four-thirds image standard developed by Olympus, Panasonic and Leica. Other: the image color balance and quality are outstanding, the operation is intuitive and clear, it holds two slots for CF and XD cards, battery life is very decent (better than the E-410), low image noise at high ISO values...

Here's a good and very informative website that covers everything about the whole Olympus E-range:

http://www.wrotniak.net/photo/oly-e/index.html

Good luck!

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