This video explains difference in layout between DDR3 (fly-by topology) and DDR2 (T-branch topology).
Best picture I have ever seen to explain memory length matching requirements. Have a look and think about that.
This video explains how to do DDR2 / DDR3 length matching in Altium Designer.
This video explains how I was doing DDR3 layout and length matching in Altium Designer.
DDR3 mirroring is a feature of memory controller. This post explains how it works.
Download PCB brd files and see examples of DDR, DDR2 and DDR3 layouts. Pictures show difference between DDR2 and DDR3 address routing.