I have the A620 and now the G9. The G9 will create more noticeable noise more quickly than the A620 as you increase ISO. The A620 only goes to ISO 400, while the G9 can shoot at ISO 1600 (3200 in scene mode). The G9 noise is very evident compared to the A620. However, there is a lot more to like about the G9, and the images at ISO 80 are fantastic. The higher MP allows me to make poster sized (24" x 36") prints that are excellent IMHO.
I have also used the ISO 3200 option to take photos by candle light (handheld), something the A620 cannot do. The images have noise, but at least the option is available. You will have to become familiar with the various noise software packages on the market, and then learn how to use them without seriously degrading the picture. Here is an article on the G9 and noise;
http://www.photoaficionado.com/tips/noise_g9.html
I try to shoot as much as possible at ISO 80, and then not have to deal with noise.
Image stabilization is nice, but I find I still need a tripod to get my tack sharp results. The 6x optical of the G9 is nice, but I also use the Canon tele (2x) converter to give me 12x optical. It is good to have on occasion, but I don't use it often.
The A620 images are very good, but I see a much better result with the G9. Plus, I feel the G9 gives me all the control over the process I will need, short of having a DSLR. The A620 and the G9 are almost the same size, so I am happy I have not had to increase the physical dimensions of the camera. I did not like the neck strap of the G9, so I purchased a wrist strap the same size as the A620 strap. I got it here;
http://www.coolanyard.com/1/thin-line-black/#1
There are many others to choose from, but I preferred this thin strap.
We kept the A620 so my wife could use it. I still play with it, but mostly to teach her how to use it. I now prefer the 3" LCD of the G9 to the 2" LCD of the A620, but I miss the articulating LCD design. Many G9 owners would like the return of this feature to the G series.
Dennis