See this Article from an SC440 owner in Japan:
Use page translate.
He was using PC2-5300 ECC Unbuffered RAM. I will try PC2-6400 ECC Unbuffered 800Mhz RAM. Hopefully, it is backwards compatible to 667Mhz and 533Mhz.
I already installed 8GB PC2-5300 ECC unbuffered RAM and it works just fine in the Dell PowerEdge SC440.