I have some experience with 28.8k dialup

But not much with 26.4 (but it shouldnt make a diff?)
Right what you have to do is lock down the frames per second/com_maxfps as low as you need.
I find 40 works for me, although some days I take it down to 35.
That has the biggest effect. Since cg_physics (q3 1.32) your fps shouldnt
make any diff to your bounce lengths.
Rate, low as it will go, 2800 for you should be fine but maybe 2500.
Snaps, low as you can, 20 should be fine.
cl_maxpackets, 20 again.
That should do it, works for me in all quake 3 engine games

)
So to summarise, in console type in
/cg_lagometer 1
so you can see what effect this has. You want as low, and as green a bottom line as you can get. If you can get a really flat top one that doesnt affect the bottom one too much do that as well. Then....
/rate 2800
/com_maxfps 40
/snaps 20
/cl_maxpackets 20
also if you are playing quake or sof2 or some other punkbuster enabled game (doubleplusiffinty play Trem)
/pb_sleep 500
erm might be worth turning bad packet resend on and off to see if it helps
depending on how many red lines you get
/cl_packetdup 1 or 0
(really you dont want to get hardly any, really)