* First of One:
http://h20331.www2.hp.com/Hpsub/cach...0-225-121.html
Only the x9300 supports SLI, according to this.

Well, it's a bit outdated, besides the xw9300 (which is already
obsolete) it's successor xw9400 does support SLI, too.

But SLI is only supported with Nvidia chipsets, and since there are no
Nvidia workstation chipsets (nForce Professional) for XEON but only for
Opteron there is no XEON workstation with SLI support.

Benjamin