Fly's right. I forgot that most wireless routers aren't really dumb hubs on their inside network anymore but switches.
I think you might be able to downgrade the switch to function simply as a hub in a WRT54G using DDR-WRT, but since it's a ISP provided device, that most likely is out.
If you use the vendor router as a 'modem' instead, and get yourself a cheap (because they are now) WRT54G, install DDR-WRT on it, disable switching on the 4 inside ports, and hook the ISP router to the external port, you should then be able to use wireshark to monitor all inside network traffic.
But the PFsense homebuilt router idea, overall, is most likely the one solution that will work best for you if you are looking for a quasi-business level monitoring solution for a home environment
Rather than trying to load software directly onto whatever device all your traffic passes through, it'd probably just be easier to enable SNMP on said device (assuming it's capable of that) and pull stats from that.
I'm playing around with sFlowTrend to collect just sflow data. I can see it in ironview but ironview has a shitty database setup so after you get a few hundred switches sending sflow data in, it starts to slow down too much.