Specifically the unit needs to support PPPoA, which is the point at which my older WRT54G (sadly, revision 7, termed 'not possible' by DD-WRT) is (or seems) unusable.
Essentially, the network would be used to share regular browsing and perhaps occasional media; limited-to-no internet-gaming, so while gigabit ethernet would be nice, it's not essential that the hardware support it.
Perhaps as a tangent, with the WRT54G, I have an ADSL2MUE router, which is itself capable of PPPoA; is there any way of connecting the two devices together while having the modem control/handle the connection? The set-up instructions I followed -that came with the devices- suggest that the router takes (and has to take) control of everything, which is a bit of a problem.