If that MB is incapable of seeing a large drive:
Go into the BIOS, and the Basic or General settings. On the first HD (the 20GB drive), set the Drive to User and the Type to Normal or Large or CHS (not LBA) so you can edit the cylinder count, whichever of those settings gives you 16 heads and 63 sectors and however many cylinders. Set the cylinder count to 16382. Then return the Type to LBA, but leave the Drive as User. It should show near 8020MB as size.
This will let you boot to an 8GB partition on the 20GB drive, kinda lame I know but it should get you going.