I've had good luck with sun-rbootd-1.3 on a 486 DX-2 (66). Linux 2.0.34. 
Absolutely reliable. 

It seems to be touchy on faster processors though. With a 300 MHz K-6 I have
to run it in debug mode and press the CR to get it to cycle thru the
request/respond and start to load SYS_....[ UBOOT | INSTAL ]. 

Best of Luck. 

Dave