Looking at the X8DTI on Supermicro's website it has regular 32 bit 33 mhz PCI slots and pcie slots. PCI x is a bit longer to support 64 bit cards and can run at 100 or 133 mhz. I am guessing you are referring to the white PCI slots.
The key for PCI compatibility is looking at the notch in the card. If the notch is toward the outside of the case it is 3.3v if it is inside it is 5v. Looking at a few pictures of the X-Fi Sound Blaster it is a universal card with notches on both sides and should work.