If you're doing portrait work, I highly recommend you stick with prime lenses. Although they lack the flexibility of zoom lenses, they make up for it in crisp, sharp pictures. As for zooming in and out, your feet become the primary tool for this function. This forces you to be more creative with your framing, so that you're not lazily standing in one spot and just zooming. I've found that owning the EF 50mm f/1.4 as my first lens greatly helped my sense of perspective, as I had to rely on myself to bring the picture I wanted to me.
That being said, I'd highly recommend you invest in a 50mm lens. The f/1.8 version is $80 USD, and the f/1.4 version is $310 USD. Both very sharp, fast lenses, and being on the 400D's cropped sensor, you'll actually get 80mm coverage. That focal length will bring you up close to the action, but not too close as to be too limiting. If you feel you need to stand a little further back yet still want close shots, try the 85mm f/1.8; it also retails for about $300 USD. It's equally as good but gives you 136mm focal length. The most commonly used focal range for portraits is from 50mm (full body on full frame camera) to 135mm (also on FF).
You should also note that these prime lenses I've mentioned have large, fixed apertures. The large apertures will give you much smoother bokeh (background blur) than smaller ones, as they allow you to narrow your focusing range.
My gallery has portrait examples shot with a 50mm f/1.4 on a 350D and 30D. I'd highly recommend you check out one of the primes I've mentioned. Good luck!