1GB memory ECC registered vs non-registered

[Dave [Hawk-Systems]]

Currently have some ISP1100 rackmount servers with 512k ECC unregistered PC100
RAM in them.  In preparing to pick up more memory to boost these little boxes up
to 1GB of RAM each, a memory company rep indicated that unregistered ECC would
work fine up to 512mb, but if we moved to 1GB of memory then we would have to
switch all memory over to registered ECC or we would run into problems...

comments or insight?

Thanks,

Dave