You are not going to see a drag down of the system at post due to the nature of ECC being there to error check data from a running OS as well as any program you are running. At post time the only data is the bios programming going into effect to perform the initial hardware tests followed by the search for the boot device.
If you ran an ECC DDR400 512mb dimm on a system you just ran a non ECC 512 on you wouldn't even see a lag. When you started running apps in the Windows environment then you would notice a change at that time due to the error checking taking place. That link has a good article on how companies could have easily made error checking active on most boards.
|