I'd suggest going to
DPReview.com and checking out their Canon forum. This topic (especially A610 vs A620) has been beaten up to death there.
The point PurdueRy makes is valid, however, from what I understand A610 and A620 use the same size sensor, and with A610 having fewer MPs, that makes its images less noisy. Some of the samples do support this.
I've been thinking about these two cameras myself (I'm not considering A540) myself. And frankly, I'm leaning towards A610. The difference of $70-80 would cover a fast SD card and a case (I already have 3 sets of NiMH).
But, I not big on post-processing, and cropping the images. Hey, I barely remove red-eye

In any event, definetely check the source I cited above. A wealth of information.
Hope this helps.
-- Alex