I currently own both of these cards. I use them in my Gateway sx2800-01 which only runs a 220W power supply. I have played Far Cry 2, Bioshock, and Fallout 3 with High to Very High settings. The HD4650 struggles a bit with Fallout 3's massive amounts of shading. Bioshock is not a problem on the HD4650, even on the highest settings. Far Cry 2 fares a little better than Fallout 3 on the 4650, but still suffers from having the "open world" shadow problem to deal with.
I just purchased the Galaxy 9600GT low profile low power card two days ago. It was a pain to install, but it's doable. There is a noticeable difference in my pc's ability to play Far Cry 2 and Fallout 3 now. The 9600 handles those games much better than the 4650. However, I play it with my PC's cover off to increase airflow. I played Far Cry 2 for 3 hours straight, on Very High settings, and the GPU temperature never got above 78 degrees Celsius.
What's funny is that, on my SX2800-01, the HD4650 actually runs about as hot, sometimes hotter than the Galaxy 9600GT. The HD4650 could get up around 85 degrees Celsius.
I do want to get a new case and PSU to better handle everything, but for the time being that is what I've found between the two cards. I still like the HD4650 and I'll keep it as a backup for the 9600GT. You may have found all that out for yourself by now, but maybe that helped somewhat.