I'm sorry, I don't know the difference between registered and unbuffered ECC, but I can tell you that almost every high-end workstation has ECC disabled in the BIOS. Which makes having ECC memory kind of useless.
However, I would defintely get the best memory recommended for your motherboard and processor. If it's ECC, then so beit. If it's not, fine. Just don't d_ck around with cheap memory, becaue your machine could last for a very long time. I'm typing this on a Dell PII 333mhz with 386 MB of Kingston ECC memory (because that's what was recommended for the Intel's OEM Dell 440LX motherboard). BTW, ECC is on it's default setting (off).
I didn't get cheap equipment and everything is working fine 9 years later!