this is what i know:
the model S400 have a bigger image sensor of 1/1.8" CCD, or about .56", while the SD630 have 1/2.5" CCD, or about .4" CCD.
the benefit of bigger image sensor is better low light capture and lesser picture noise which mean less graininess. But seeing the S400 is probably is a 2003 model while the SD630 is a 2006 model, in that 3 years difference. I think they probably manage to shrink down the image sensor size from .56" to .4" and still perform similiarly in picture noise and low light level. Technology improvement there.
Well, the SD630 have 6 megapixels while the S400 have 4 megapixels, but that means little to me as image sensor size is more important since either 4 or 6 megapixels are really good enough for 8 x 10" and bigger prints...
But seeing its been 3 years, so i guess you get similiar low light performance and 2 megapixels more on the new camera.
Canon is nice enough to let you decide to get the new one or wait for them to get the parts for repair on the old one. So, its your choice. As for me, I think maybe selecting the new camera isn\'t a bad idea.