Do I need to control trace impedance for DDR2 memory?
Since you mentioned that it will be low-speed then you most likely should not bother about impedance. The question is what exactly speed you are planning to go for?

My experience shows that you should think about impedance if the memory speed over 200MHz. If it's less than this number, there shouldn't be any problem, unless you have unusual PCB setup (6 layers, for example). But if you have, let's say, 4 layer PCB with two dedicated power planes, traces up to 3 inches can be even left without any termination at all. Just align the traces and you're done.

Anyway, if something went wrong, you always have the possibility to talk with memory on lower speeds.