you may be running up against the limit of your body...
Canon EOS 300D / Digital Rebel Review: 14. Photographic tests: Digital Photography Review
the 300d does have some detectable noise at ISO 400. did you start with RAW, or are these one flavor of JPG? at least the L?
the reason I asked about lighting is that the green reflection from the background also messes with your image a bit, as photokey will fade out the part of her that has the right key color on it, reflected from the background. so you have to make sure the subject is either well lit from the sides, or far enough away to control the reflection :(