I recently upgraded to an Athlon 1.33G processor on a MSI K7T266 Pro2 mainboard with 256M PC2100 RAM. I did not notice upon assembly that the RAM was ECC instead of non-parity. Crucial lists this Mobo as not supporting ECC memory and the user manual confirms this.
Would there be a performance increase (however small) to replace the ECC with a non-parity module???
No. The MCH is setting the ECC and parity checking to zero. This means that it is disabling the checking features and using the entire chip density like non-parity.
You can use ECC and Parity on non parity boards but you cannot use Non-parity on parity boards. If the board is looking for parity and can't find it, it will go on strike and not work for you until it gets what it wants. Like I stated above, on non-parity boards the error checking and parity bit checking are hardwired to zero and won't look for the extra bits.