Hi Monelle, welcome to the Cameralabs forums!
Most Digital SLRs deliver slightly softer or less punchy images than a consumer camera will using their default settings - this is nothing to do with the quality of the camera though, but the way it's been set up for its target audience.
Consumer compact owners prefer vibrant punchy images straight out their cameras, whereas DSLR owners tend to prefer more subdued output which they can tweak or enhance later on their computers.
Every DSLR offers a wide variety of settings which can apply more sharpness, contrast and saturation if you'd prefer punchier-looking images straight out the camera though. Indeed some of the more affordable models are set up like this from day-one, such as the Nikon D40.
The bottom line though is not to worry. Any DSLR is capable of delivering far better quality than a compact, and you'll really notice this under low light conditions. It's also easy to tweak the settings to apply more sharpening if that's what you want, although after you've taken a few pictures you might actually prefer the look of the default settings.
So it's not that a DSLR image is soft, but that a compact tends to apply more digital sharpening by default. Believe me, if you're after the best image quality, you'll notice a massive difference between the SD800IS and a 400D / Rebel XTi - in the favour of the latter!
But if your budget can stretch, you'll get even better results by upgrading the 400D / XTi kit lens to a better model. The Canon EF-S 17-85mm is a great choice and also includes Image Stabilisation. In fact we're updating this lens review today, so head over to http://www.cameralabs.com/reviews/Canon1785EFS/
to see how it compares and for a new video tour!
Hope that helps!