Just be careful with allowing JUST TCP port 80, because you also require DNS on UDP port 53 for HTTP requests to work properly.
I would also suggest that you allow HTTPS on TCP port 443.
The manual for that router is terrible, because it does not show any firewall options, besides the port forwarding ones. Typically you'll add the following firewall rules in the following order:
1) Accept destination UDP port 53
2) Accept destination TCP port 80
3) Accept destination TCP port 443
4) Drop everything (else) - unless you can set the default Input chain action to Drop, in which case this rule won't be necessary.
A last resort could be to use something like OpenDNS with categories, where you only allow certain categories - like education, business, etc, and block other categories like gambling, nudity, proxies, etc.
Oh, and DD-WRT has a very good firewall, which would most definitely allow you to do these things. With DD-WRT's firewall, you can even allow 1 person access to everything on the Internet and block everything except for HTTP(s) & DNS for everyone else...
