The 1.6v should be more expensive then the 1.65. This is because it should be better ram, as it has to do the same thing with less voltage.
Asking this question is a bit like asking whats the difference between car A that gets 30MPG and car B that gets 28. The answer is that 1.6 or 30MPG will cost you less over the long run. Other then that there is none.
No. The less voltage you need to do the job the better. The system will consume less power overall, saving you $$$ on your power bill. Realize that the difference between these is almost nothing, so we are talking about pennies per year. As I said in my first post, from the end user stand point there is no difference.