The WRT54GL hardware is incapable of much more than 30Mbps regardless of the firmware, because the LAN and WAN ports are all fed by a single 100Mbps ethernet interface.
DD-WRT's IPv6 support is notoriously abysmal. The only answer you'll get is "we turned on the kernel module, you can implement the rest."
I'm currently using a WNDR3800 running OpenWRT with Comcast IPv6, and that works pretty well.
Also, the current line of Linksys routers all support IPv6 in the stock firmware. Presumably they test it with Comcast, but I haven't tried one myself: