@andz: from wat i understand, the RE3 is meant to be used with tri SLI/XfireX on dual slot VGAs. To go with 4 way SLI/Xfire you would need at least a 9 expansion slot motherboards called XL-ATX form factor. This is probably why ASUS didnt bother putting 2 NF 200 chipsets since it's only standard ATX form factor.
regarding Xeons, i think most high end boards can support Xeons but not all of the Xeons.
Here's the compatability list for the UD9:
GIGABYTE TECHNOLOGY Socket 1366 - Intel X58 - GA-X58A-UD9 (rev. 1.0)
Most of the compatibility is with Xeon 3000 series cpus though.
Glad to help
