I know that i can use PXE to boot the server, but is it possible to boot the server from the network and than tell it that he need to use some kind of network disk for its os? So that i don't need any mass memory on the server (hard disk, usb drives..)
Is it possible? Thank you a lot!
Yes, you can tell Linux to mount an NFS partition or NBD as the root partition. There's likely to be support for a lot of other network filesystems and block devices too, because the initrd can do magical things; NFS and NBD just happen to be the two that I'm most familiar with in this scenario.
This is a good write-up of how you might do it for NFS, while Christian Laurs has an initrd builder you can use to get started with root-on-NBD.
Yes this is possible. For most operating systems. Though with Mac OS X I'm not sure.