ISDN should be ok.
Strangely enough there are some modem players who seem to do ok (although lagged). This probably has to do with the fact that most 56K modems run far less than 56K (33K is average for North America, but elsewhere can get far worse).
The only way I've seen to really avoid lag is to have all players with good connections (384K+). Also, as EU2 seems to send seperate packets to each player, a good outgoing bandwith for the host is critical (ie: ADSL is probably not ideal, unless it's like 1.5meg/784K).
Anyway, it all depends on what you consider playable. I don't really have a problem with playing lagged, but it does make retreating from combats in time nearly impossible.
Waffen