>1- Do you think is relevant the amount of people that are still not able to use 1024 ?

It depends on your application. In most applications I work on the forms are designed for 1024x600 or 1024x768. We have a resizer control on the form, though, so that users with lower resolutions still can see the entire form albeit smaller. For applications that I primarily meant to run on notebooks, I would still go with 800x600, though.

>2- Is true to think that all people using windows XP or higher can change their screens to 1024 resolution ?

Or even less, although Windows XP has a minimum resolution of 1024. Some dialogs are not entire visible with less than 1024.