Enabling ECC on the Asus T2P4 does cause a performance hit as
commented by someone (somewhere back in the mail).  The hit is
caused by DRAM read timing of x333 instead of x222 with 60ns 
memory.  Can you use ECC and manually set x222?  It would
appear so but I have no idea what problems this might cause
(if any).

lmbench reports that the memory access time jumps from 180 to
258 ns.