The A550 gives you a higher resolution LCD, an available "speed priority" drive mode with 7fps if you don't need AF between frames; a larger internal buffer size (which could help to get more photos in a row before a slowdown shooting raw using continuous drive mode if taking lots of consecutive bursts), and a slightly higher resolution sensor (14MP with the A550 versus 12MP with the A500).
But, most users would be fine with the A500 instead of the A550. Image quality is very comparable between them, and neither camera is a "slouch" in the performance department.
I don't know if it is helpful or not, but the price of the Sony A-500 body has been consistently dropping over the last three weeks. Today (08/24) www.amazon had the A-500 body at $(US)484.97. Forty five days ago, that A-500 body was over $(US)600.00.
