I need a recommendation for something that I can use to log all
packets going through a choke-point.

The layout would be Router1 <-> BOX <-> Router2

Router1 is on the periphery, and the Router2 is a wireless router that
is also the DMZ area.

I need to log and then be able to analyse all traffic that is coming
through Router2 in either direction - I am figuring that logs in, say,
CSV format would be ideal, but anything machine readable should be
okay.

I am thinking that the BOX will be a dual-NIC PC (we have access to
plenty of machines for this) that we load up with whatever OS is
required (we have spare WinXP licenses for example, or could be, say
(K)Ubuntu).