Thanks for the reply! The controller card chipset has 3114 silkscreened onto it and onto the PCB, and I got the x64 base and RAID drivers for the 3114 from the silicon image website. I got the 32-bit Win7 drivers also just for testing but (of course) the installer doesn't see those no matter what. For what it's worth, the SATA controller BIOS loads fine and reports the new drive capacity correctly, it's only the Win7 installer that can't seem to figure out what to do with it.
Unfortunately the onboard SATA implementation is broken (1st generation VIA SATA chipset) and will crash the computer at boot if non-SATA150 drives are attached to it.
I'm kind of surprised there aren't generic storage drivers on the install disc, it wasn't an uncommon chipset for SATA RAID in the K8 era