Dual band just supports both the 2.4 & 5Ghz ranges. Many crowded environments have very little space left in the 2.4Ghz band. That will have a big impact on performance and in that case moving to a (usually) less crowded 5Ghz band will make things much better. The newer wireless AC is also 5Ghz only, and does have longer range and better throughput than wireless N, so if both your router and your device support Wireless AC, then you can expect better performance.
5Ghz wireless N though isn't necessarily any faster and will actually be lower range as the higher 5Ghz frequency is more affected by walls and the like.
What's your current setup and is it not fast enough?