My recommendation is to use one of the many pre-made Linux router/firewall distro's out there. My personal favorite is
IPCop. Others to consider are
SmoothWall and
ClarkConnect.
I would suggest using this as your firewall and router, and not using your Linksys device as a router any more (you'll probably still need to use the switch function of it, or buy another switch). I've never found a store-bought router that's been anywhere near as stable as my linux router box. These packages are pre-configured for what you need to do, and the instalation guides you through it, so you don't have to be a linux guru to set it up.
IM, PM, or email me, if you want specific guidance on selecting one of these, or setting up/configuring IPCop, or my experiences with IPCop.