I use a SB-800 speedlight and make use of its IR AF assist beam. It helps a great deal and I do not shoot without it in dimly lit situations. The IR AF assist can also be provided with the SC-29 cord with the flash on a bracket. There are places that will shorten the SC-29 cord so it is easier to manage on a flash bracket.
I've been following this over the past few weeks and have learned quite a bit... I really need to stick with the center focus point on the D610 and recompose, and I will be much better off with a D750.