In most cases, if one router is giving you weak signal somewhere in the house, you are better off using two... one in regular wireless router mode and one in repeater mode. This requires an ethernet cable be run between the two.
If your current router is weak in some area of the house, a new router MAY get a little more signal to your weak spots, but it will be a marginal improvement compared with adding a repeater somewhere near where your signal is weak. It does do something though, I have an AC68U and the 2.4GHz band of this router is consistently higher signal and throughput than the 2.4GHz router my U-verse provides. ~10dB better signal at our weakest location in the house. However if I go outside to the fringes of service, the two routers are not as different (but the AC68U is still higher signal)