Hi CK,
I wouldn't go as far as to say the S3 always gives better image quality than your IXUS/SD, but it's certainly an impressive camera. However please consider the bulk of the actual unit. If size is an issue, you could get buy with something like an A710IS, which boasts a very similar feature set, minus the optical zoom superiority of the S3. The best thing I loved about my S3 besides the absolutely fabulous level of control was the swivel screen, and excellent movie capabilities. The S3's movies were of stunning VGA quality, combined with stereo sound. The dedicated movie button on the camera is very useful.
Canon usually announces new products around Mar-ish & Sep-ish (i.e 6 month intervals). That isn't to say an S4-IS will necessarily be announced in the next few months, but if it is, it would be around Feb/Mar (along with all their other launches).
Good luck,