ASUS P5LD2-VM DH does work.
ASUS P5LD2 does NOT work.

You might also try the following with your current configuration:
From SCO Knowledge Base.
The use of multiple processor cores is enabled by default
on OpenServer 6. To disable it, enter the boot parameter:

MULTICORE=N

at the boot loader prompt (or add it to the /stand/boot
or /etc/default/boot file). If the use of multiple processor cores
is explicitly disabled, then the use of hyperthreading is
also disabled.

Note that on some systems (particularly where multi-core
processors are included in the system's MPS tables),
ACPI=Y must be entered in addition to MULTICORE=N to
disable the use of multiple cores.


NOTES:

The boot parameters above are discussed on the boot(HW)
manual page.

Boot parameters may also be set using the SCOadmin System
Startup Manager.

Having multiple core support enabled has no effect on
systems that do not have multiple core processors.