I want to build a new computer with Mandriva, SuSe or any Linux desktop distro installed, but I get the feeling that no 775 motherboard in the marketplace is supported.

So then I think go with Intel chipset P965+ICH8R and Why because the Mandriva, SuSe compatibility
lists are full of Intel chipsets and the P965 has an improved memory controller.

So then I read a review of the P4N-Sli with nForce4 chipset, which works flawlessly under gentoo Linux, and I think.
P4N is good
But I think
How do I know it works with SuSe/Mandriva distros? after all there are no Nforce chipsets in the compatibility lists of SuSe/Mandriva.

Do people have motherboard compatibility issues with Linux or should I just feel free to buy any motherboard I like?

I think the problems that you will run across deal more with integrated components. I've got an Intel DG965RY mobo, and it's run flawlessly with every live distro I've tried [usb distro, my cdrom drive doesn't work in Linux - but that's not the motherboard's fault]. The integrated ethernet hasn't worked, and I have yet to try out the integrated sound or video. The integrated video drivers are seemingly being worked on at intellinuxgraphics.org. It detects my SATA hard drive and I can use files from it. And obviously the usb works perfectly.

I'd wait for someone to answer who's more knowledgable on the subject just to be sure, but I have used this board first-hand and have had no problems with it [except you know integrated ethernet].