Most of the time people have pppoe since atm is very expesive. the mtu should 1480 for xp, or 1492. Now for some odd reason though like bell south like the 1454 why I don't know like you said probably less padding. With pppoa I was told and read it's 1500 for mtu. However it's not really the modem that caps the speed it's more the network that does that. Actully it's funny a dsl modem isn't really a modem it's mutiplexer.
Each provider's network is different best bet to ping to find mtu for your network.