It helps to read Jeff Keller's reviews and look at the sample pictures.
The SD450 is intended to be a stylish ultra compact, the SD500 a stylish ultra compact which is a bit bigger to accomodate the 7mp sensor and a bit better lens, and the A6xx is intended to give you as much camera as possible for the money (even better lens, manual controls and swivelable display).
So, it is up to you which one to choose, what you find more important. Image quality wise the A610 and A620 perform a bit better,since their size allows for a better lens.
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Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30
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