1) Going for a new router and a firewall
2) Going for router with built-in firewall.
I wanted to know which option would be economical.
One last thing, what's the difference between Cisco pix 515 box and IDs box.
A router with in-built firewall will be economical option but then you will loose on processing power of the router, you will not get a statefull transition in case of failure and you might have to consider interoperability of different features you configure on the router.
A router with a separate firewall will do all these - but will be costlier. Just remember, routers are meant for routing not filtering and firewalls are meant for filtering.
IDS is a host which just collects all sessions and analyzes them for illegal or unauthorized connections and will then report about them.
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