The Gentoo livecd comes with both the siimage (IDE access) driver and the libata (SCSI access) drivers for the 3112 controller. It's safe to install Gentoo using the siimage driver (and accessing the drives as dev/hd*) and then building a kernel with only the libata driver (so you'll drives will be called /dev/sd*), but please do keep that in mind when you create your /etc/fstab. I have a similar setup (NF7-Sr2, Barton 2800XP) and it now works pretty well.
The installation handbook is pretty thorough and walks you through the setup, including installing and configuring Grub to dual boot with Windows. What part was it that you didn't understand?