I agree with Osiris 30 that we should not expect the noise will be substantially better just by switching to a newer processor. I have both the 20D and the 40D. The High ISO (1600) Noise on 40D is worst than the 20D by a very very slight amount even the 40D have a newer processor. In the semiconductor business, sometimes a newer product may be just a cost cutting ( by going to smaller geometry and hence ends up with a smaller chip) move rather than a performance improvement. To be fair, by going to smaller geometry, there may be a gain in the processing speed of the processor.
Therefore, the real improvemnet in noise is by improve the sensor of the XSi ( Gapless microlens, shorten the metal connection, amplifier built into each pixel etc. which is already done on the 7D sensor) and use it on the T3. Canon may end up doing that in order to out perform the Nikon with the same pixel count sensor. That is my wishful thinking.