This is exactly what I was looking for:

I need to have completely stateless PXE bootable linux (ubuntu in my case). The system needs to mount one NFS share after booting to keep config files and other stuff, but NOT run linux's root file system from it. I already have PXE bootable machines which use NFS share as root, I need to go one step forward.

I need it to be able to share single, small size boot image (no more then 250MB) for many PXE booted machines. All files required to run the OS need to be packaged in the initial image which will be copied in the ramdisk, mount root filesystem from it and run necessary scripts after mounting the root.

If anyone can direct me into resource/documentation which I can use I'd greatly appreciate it.