>I still use 800 x 600 to configure my softwares
>
>But now people use 1024 X xxx resoltion I they see a large amount of screen wasted
>
>I am considering change my softwares to 1024
>
>1- Do you think is relevant the amount of people that are still not able to use 1024 ?

Not really. Only people using your software are relevant.

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

You're asking wrong question. Even if people can change resolution doesn't mean that they will be willing to do so.