rugbert, I'm glad it worked.
The "root (hd0,0)" grub command tells GRUB to mount /dev/sda1 and look for files there.
tells GRUB to look inside the filesystem on /dev/sda1 for a file whose path is /boot/vmlinuz-2.6.20-16-server. You were getting Error 15 (File not found) because vmlinuz-2.6.20-16-server is inside the /boot directory.
kernel /boot/vmlinuz-2.6.20-16-server root=UUID=6e69ca5e-3407-4afd-835d-2e1b7c019cb7 ro quiet splash
The next part I'm a bit shady about, but I'll tell you what I think is true. I'd appreciate it if anyone can confirm this or correct me:
The kernel needs other files to complete the boot. Some of those files might live in /etc for example. The "root=UUID=6e69ca5e-3407-4afd-835d-2e1b7c019cb7" tells the kernel to look for these other files on the partition whose UUID is "6e69ca5e-3407-4afd-835d-2e1b7c019cb7". You can find the UUID by running "blkid".
The "ro quiet splash" are kernel boot options sent to vmlinuz-2.6.20-16-server.
ro means "Mount root device read-only on boot". I find this a little strange, since the root filesystem is not read-only by the time the boot completes. I guess it is remounted later, but I'm unsure.
quiet means "Disable most log messages"
splash means display the orange progress bar instead of text messages