Provided you've disabled remote management on the DSL/Cable router, and you're routinely updating your SyGate firewall & discontinue use of IE (as was just mentioned), I'd see no reason to replace the router. Are you doing any Port redirection or hosting of systems to be accessible from the Internet?

The next step up would probably be something like a NetScreen 5xp or similiar device, but you'd still need a DSL/Cable modem.