Check out Firewalla.

It’s a combination security appliance -think a router with IDS, IPS and firewall capabilities.

I have it and love it. After coming home from working on IT all day at my company, I just want my home network to work. Firewalla does that, but with all the extra goodies that an IT guy could want.

It also has a “monitor only” mirror mode if you really want to keep that Asus router.