X -broadcast: resolution too low at 400x600

[stth]

Hello,
I am using puppy linux for some days.
I have the standard iso with usbstick and enough RAM to load it into.
So far everything is well.
The machine where puppy is on should work as a kind of client to access my linux server.
With knoppix I booted to boot prompt (runlevel 2) then started with X -broadcast
Knoppix is way to fat for me so I tried puppy.
I have configured my displax with 1024X768 which went very good.
Afterwards I exit fvwm with 'exit to prompt'.
Then I tried to start with
X -broadcast
It also starts the kdm login on my server but the resolution is very bad (kind of 400x600)
Do you have any advice for me?
Thank you very much in advance
Stefan