Experiences with Ubuntu + external display?

Do you have experience with using external display under Ubuntu?
Can you suggest a model that worked well?
I tried a bunch with my current Tuxedo notebook, but i was never able to set the resolutions so i can use both screens (laptop+external).
What parameters should i look out for, when i’m searching for an external display? Ie, should it have the same PPI?

Thanks in advance!