Try changing the grub boot line to use root="LABEL=/" instead of hardcoding the device.

I'm guessing they've made the classical blunder of using a different kernel for run-time than the one used for installation.. different kernels mean different devices detected etc.. and different device addresses.

Modern distro's cope with this by using filesystem labels (eg. the line above) instead of hard coding drive name strings..

[EDIT] It's either that, or a missing initrd= line in the grub config.[/EDIT]

If this is your first serious foray into Linux, then dump Gentoo and use (K)Ubuntu instead. Much much MUCH more pleasant.

Cheers