I want to buy a new computer and i have founded the right,
it has 7.1 sound onboard but i have new 5.1 speakers.
Is there a solution to get the 5.1 speakers with 7.1 output
working, or is it better to buy a 5.1 soundcard cause the
7.1 sound onboard is bad?

Sound cards support up to and including their number.

I would say save your money and a pc card port, use the 7.1 on-board sound card. The 7.1 will fully support the 5.1 speakers and gives you the option to upgrade to 7.1 speakers in the future.

It sounds like you are building your own computer. If so, you can always go out and by the 5.1 sound card if you don't like the motherboard's sound quality. There is no harm in trying the on-board sound before deciding to buy a new sound card.

Thank you i wil use the onboard sound first, i dont build it self, its a
configureable "instant-pc"