I've had the exact same problem on my nForce 2/3/4 S939 systems. I solved it by making an Amithlon CD with the updated kernel, and adding "mem=512m crusoe" (without quotes) to the Amithlon startup script on the CD. (without the "crusoe" I'd only get a black screen)
My Amithlon CD startup is like this:
bzImage init=/linuxrc console_level=0 root=/dev/ram0 initrd=bigird.gz vga=769 ramdisk_size=12010 leavepages=10000 mem=512m crusoe
Don't ask me where I found the "crusoe" thingy, its a while ago, I don't remember
Good luck - Mike
