Registered NON-ECC is available in some speeds. THAT can be used, and you can take advantage of the mfring QC behind the Registered RAM. ECC features will not work though, the non-ECC supporting BIOS should ignore the extra module or modules-- in some cases, on non-ECC supporting baords, the sockets themselves do not have spring contacts for the ECC pinouts either. Whether Windows or another O\S will ignore ECC, in this case of using ECC on a board that does not support it, is another story entirely. And therein does lie one BIG problem with using ECC on a non-ECC supporting board: If Windows tries to use ECC features, and board cannot provide underlying logic for that, then you can get a mess of a RAM-use mishmosh as board tries to use the extra ECC module for RAM or igonres it totally and Windows tries to use it for ECC checks. You will get mapping errors that lead to RAM violates and BSODs in O\S out of this, and you will get RAM size mismatches between BIOS and Windows.

But it is not a fiscally sound thing to use either, right. CAN use registered non-ECC but not worth spending the bucks in most cases unless you soon plan to change abords to one with Registered RAM recognition in BIOS, and ECC or Registered plus ECC is not a good thing to use on a board that does not have BIOS support for it.

Real old boards and early PC gens ECC was an upgerade option for mots of boxes. BIOSs could use either. A BIOS that can ECC recognize and an OS that can map accordingly are major requirements for ECC, plus tight power cycle timings and voltages on whole board.

I would slightly mod what MM said as to registered-only RAM, but as to ECC do not use unless your board supports it, whether registered plus ECC or non-registered ECC which is a questionable thing to offer or use these days anyway compared to higher end Registered plus ECC. Registered feature is mostly a quality assurance thing, and used mostly with servers that use ECC for data integrity assurance on hardware. And ECC plus registered RAM needs tighter sync ratios to work right and sync to CPU than other more generic but high quality RAM does-- for pure Extreme OCing, would not use registered plus ECC, period. You will get a system that is very picky about what combos of timings it will accept if you use registered plus ECC-- but much better assurance of integrity of data in RAM at correct timings for RAM and CPU.

We have ignored BUFFERED RAM here, that is something else having to do with using physical (usually smaller) modules as temp storage right on the stick of RAM. Some ECC can be physically also buffered RAM (and I have seen buffered non-ECC at certain time sin history, in the age of SIMMs as best available, and BIOSs that could detect buffers on sticks). That can give you a RAM to bus latency decrease potential if the stick is QC'd right at mfr. I would not bother buying buffered OEM grade RAM. Expect to pay for QC checking of guaranteed buffered RAM.