Most of the routers that can be re-flashed to DD-WRT or OpenWRT should meet your requirements. This may void your warranty. Something with 8MB or more flash, and 32MB more memory should work well. Currently Atheros Wifi cards are best supported.
At least one manufacturer is distributing DD-WRT firmware and is open to re-flashing with alternative software. See the OpenWRT Table of Hardware for options. I run an ASUS WL-500G Premium providing DNS, DHCP, IPv6, and firewalls for both IPv4 and IPv6.
- DHCP and DNS are provided by DNSMasq.
- IPv6 is provided by a 6to4 tunnel to Hurricane Electric
- radvd is installed for IPv6 auto-configuration.
- Firewalls are implemented using shorewall-lite and shorewall6-lite.
- I have OpenVPN installed, but not configured.
Comcast appears to be one of the few North American IPSs that is working on IPv6. They have been working with the OpenWRT to release code for a dual stack home router. They have been testing implementation and deployment IPv6 using 6RD tunnels. This helps bypass problems with cable modems that don't yet or can not support dual stacks.