Hi. Just boot with tftpboot.img, that should get the installation going (if not, you probably haven't gotten your tftp/rarp/bootp configuration correct). After installation, replace the tftpboot.img with the linux-a.out kernel which actually has nfsroot support nowadays. Both images is in the disks-sparc directory of your favorite debian mirror. You might have to give kernel paramaters to get the ip routing table correct, I never got the ip configuration to work properly over BOOTP. I hope this helps a little, if not, try to be more specific what you're after.
Regards, André Johansson
