Intrusion detection?

Hi, what are the recommended tools to add to a (dial-up) NAT gateway to detect attacks on that gateway? I am not (yet :->) looking for tools how to detect things after the deed is done, but for some kind of an early warning system (and I am not particularly interested in reading the raw logs emitted by ipchains). http://www.securityfocus.com/ lists a couple of things, but this seems to be just an *unreviewed* long list of tools, with rather unknown quality. TIA Stefan