PfSense works nicely too - combined with a quad-NIC and VLAN trunks to a decent switch. With any old x86 hardware, you will have all the routing you will ever need.

The WRT54G has limited hardware resources by todays standards, albeit a classic Linux box. Even with DD-WRT though, with enough concurrent connections, the box will crumble and the packet loss will drive you insane.

PfSense also makes management of multiple subnets, much easier.