looking at the ASUS RT-AC1200G AC1200 Dual-Band Wi-Fi Router with four 5dBi antennas and Parental Controls on newegg
modem at the moment : arris sbg 6400
Yep, that is both dual band and supports AC. It should be a large upgrade over your current configuration, however:
You'll need to keep using your current router as a modem, since the Asus router can't directly act as one. If your current router doesn't have a modem mode, it might take a bit of configuring to properly configure it on your network.
If you only need AC access, you might be better served with an AC access point instead of a full router, as it will cost less and be easier to set up.