AMU-2405G-SMA---Zigwire
AMU-2405G-SMA 802.11g/b 500mW , AP/Client/Repeater/Bridge

It does not say it will act as a router so what would you intend to use?

Since there are no readily available manuals and I can't find it on the FCC web site I can't really check any specs. It would be nice if Zigwire could produce an FCCID No for the item so it could be checked in their database. If you come under the FCC reg's then zigwire will have to provide s/ware to ensure you can only use Ch's 1->11 and not to Ch13(minor point) and it should really be FCC certified(not just conforming to the FCC reg's).
I don't know why they recommended a switch as a router would be required if you are not going to use the WRT54G for that function.
edit: Does your modem have NAT and DHCP capability? A switch should work in that case.
Did they quote any prices for their eqpt?