Till i got Broadband last year i used to play DOD and Team Fortress on my 56k dial up and thought that was ok. Must of played a good year or so before broadband with pings of 350-400 and that was still playable but put you at a big disadvantage with the low pingers but improved my skill i feel
As Sinzer says you should be able to achieve pings of under 150 but even 200 on a connection i would be happy with that.
Yeah midband/isdn ok for for online play, played CounterStrike and DoD using this connection for about 2 years and plays fine, only problem is when there are patches for the game that are 50-60meg which take a age to donwload. You can also use ISDN for Xbox live as well if you want, works well for most games Project Gotham, R3 etc, but as said above don't host anygames, the slowdown/lag is awesome!!!!. like playing in slo-mo