The real question is, is your MoBo Zero Channel Raid compatible.
"The chip features a 66 MHz, 64-bit PCI interface and a 133 MHz, 64-bit PCI-X interface".
Sounds unlikely. More likely it isn't implemented in the BIOS.
AFAIK, that needs a specially designed MoBo. Is your's?
And likely does not have PCI-X either.
The same bus that the Adaptec chip is likely connected to.
It's not. Only if connected to a PCI-X bus.
64x66 = 533 MB/s minus overhead
2*U320 = 640 MB/s minus overhead
SCSI's overhead is bigger than PCI's, that narrows the gap.
And you'll only go into that territory if your devices actually need all that bandwidth.
Read the specs?
If they were to be on the same bus they likely be 32x66, not 32x33.
In your 64 x 66 bus slots, yes.