Heh, I did read that befor I posted but don't quite understand what it means, in real everyday terms. Is the error a real risk? Is it just this ram that makes me lose it, or is it going to 24 Gb in principle ?
The RAM will work and it should show up as 24GB assuming no modules are defective.
Not knowing what you do, it's hard to say if ECC is critical for your use. For people they can't tell the difference and it won't really matter.
Let's say you bought a dead-bolt for your door and the instructions call for a reinforced door frame to maximize security; in this analogy you're just going for a regular door frame. You might never need it, or one day you might.
So for ECC it's another layer of reliability on your Mac Pro.
Let's look at OWC:
You're spending maybe $15 more for ECC memory (converting 120 GBP to 185 USD), so after spending a lot of money on a computer that's multitudes more expensive than the memory, what's the real saving of $15?