We sold an X8DTU and RSC-R2UU-E8E16 with a Quadro FX3800. Do you foresee a problem with this as the latest motherboard brochure specifically states no GPU support.
Correctly this motherboard can support GPU card but may need change the BIOS setting for riser card.
Advanced --> Advanced chipset control --> North bridge configuration --> Slot SXB1 --> X16