If you go to the Mackie website (discontinued products) you will find "TIPS" on upgrading RAM. You may find all the technical info you need there.
From Mike Rivers Book "The Last Mackie Hard Disk Recorder Manual" (pg 47-48) he states "The motherboard takes PC-100 SDRAM memory modules (sticks) in the 168-pin DIMM configuration". "Memory designated "PC-133, PC-100 compatible will usually work". 256MB is the maximum size usable by the HDR..
Yes it does matter, a lot actually. You want non-ECC. ECC is self-error correcting server memory. It might accidentally work (as sometimes this happens) in the HDR/MDR board but your best bet is non-ECC, as thats what's prescribed.