Your A85 has a sensor size of 1/2.7" and the S3 has a sensor size of 1/2.5" - so the S3 has a slightly larger sensor. However, the S3 is packing in 50% more pixels - 6mp vs 4mp.
To put it mathetically... the A85 sensor is 5.371 x 4.035mm, the S3 is 5.76 x 4.29mm. The area of the A85 sensor is 21.67mm, the S3 is 24.71. So the S3 sensor is about 14% larger.
So... cramming 50% more pixels in an area 14% bigger means that you will be seeing more noise with the S3 than with your current A85. So, you're going to a camera that you can expect to perform worse, not better, in low light. What you'll gain is a more responsive camera with a lot more zoom, but at the expense of image quality.
If you're considering a DSLR, know that you can probably get one for cheaper than you think... for example, the Pentax K110D can be had for just under $400 after a $50 rebate. The current rebate ends today but there will probably be another one in a few weeks. You won't have the long zoom of the S3 (at least, not until you buy another lens) but you'll have a DSLR and everything that that entails - not only much more control, but a sensor that's nearly
15 times larger than the one in the S3. That means significantly better image quality especially in low-light situations.
Of course, if you spend a little more, you can get the K100D instead of the K100D, and get image stabilization, but that's another $75-100... it's very easy to get nickel-and-dimed with DSLRs. You'll want more lenses, then a nice tripod, plus a nice bag, maybe some filters, remote shutter release, more memory cards, etc, etc... it's a slippery slope.
