Screen resolution on Remote Desktop machine?

My idea was, to create a dedicated computer for my future CNC milling machine, which I only have to plug in LAN and power.
That way I don't need two mices, keyboards and screens.

After some trouble installing ubuntu on that old machine, I got 6.06 working.
The newer ones can't boot from my stick.....sending some errors.

I configured remote desktoo and auto-login that I can do all the final steps from my computer over VNC.
But when I turn on the milling-computer, the speaker beeps two times (I guess it's the screen-detection).
And when Ubuntu 6.06 is loaded the screen resolution is fixed at 640*480pixels.
That's not fine for working. But I can't change the settings from the screen resolution panel.

When I plugin my monitor, mouse and keyboard, I can set a much higher screen resolution.
So it seems to be a safety-setting in ubuntu.

Does anybody have any idea how to solve this problem?
As I never workes with EMC2 before, I guess 1024*768 would be okay. Or even 1280*1024 or something like that.
My computers screen works at 1650*1080.
So everything below would be fine....but at least I need 1024 (or if EMCS requires more.....please tell me!).