<< You were all playing CS through your router to a server on the net? or your own server that was allowing other internet people to connect? What settings were you using on these 5 computers? rate? etc? >>
We connected to my friend's 16 player server, so I wasn't hosting. However, I have hosted a 6 player server on my DSL line before without too many problems at all (even behind my router).
What settings on the systems? Nothing special. A mixture of Win98, Win98SE and Win2k clients, all with rate at 9999.99. Half-Life's dedicated server has a cap at how much it'll upload at and its around 6000 (~6k/sec), so we made sure to max ours out for a local connection just in case. My router is a netgear rt314 and all machines were connected to a switch (both the RT314's switch and a second switch I have on my network). We were all about 50-60 ping (we were in Southern California and the server was in Berkeley) and it was just fine.
My DSL's uprate is the key -- its 384k/upload. Without that much upload, I couldn't host that many people. With 128k/upload, you can do about 2 people just fine -- 3 people is pushing it.
vash