Continued from http://shadowphiar.livejournal.com/61524.html...It's working! It's working!
(well, mostly...)

Installing Leopard was surprisingly little trouble (once I had got the efi-x plugged in properly, which took a couple of attempts. Even with the little extension cable they give you, it's very fiddly to reach that corner of the motherboard, so at first it wasn't quite in far enough and didn't do anything).
The things I expected trouble with (e.g., system sleep, updating graphics drivers etc.) have been no problem at all. Instead, I get Time Machine falling over when selecting a local SATA hard disk as the backup volume, because "the built-in network interface could not be found". (Don't be silly, of course it can - it's working perfectly and I'm posting through it right now!). Allegedly this can be fixed buy buying a PCI network card... (or maybe a future efi-x firmware update) (or maybe it will only work on motherboards with two network connections -- it's a bit difficult to tell when your only official source of information is also filled with random people posting questions and unreliable opinions).
And then there's all the whole business with AHCI (new command protocol for SATA hard disks). One the efi-x forum there's very clear guidance that AHCI should be turned on in the BIOS after installation of the OS. What it doesn't say, is that doing so will prevent DVD burners from being seen! Aha - I think - there are two SATA controllers on this motherboard (southbridge has six channels, and a separate chip has other two) and they can have AHCI turned on and off separately. So I'll put one of them into legacy mode and run the DVD drive off that? No - because the separate SATA chip doesn't work with efi-x.
This must be some strange new usage of the words, "all features of these motherboards are supported", of which I was not previously aware.
It took a good deal of searching and browsing on their forum to discover that last bit of vital information! I've been unimpressed with efi-x's documentation and support in general. Anyway - in benchmarks I've been barely able to tell the difference between AHCI on or off, so I'm leaving it off in order to get the benefit of a functional DVD drive!
In short, it's been a bit of a faff (and that's not over yet). However, it was at least mostly quite interesting faff, and considering that to buy a machine from Apple with as high specifications as this would cost nearly a
thousand pounds more than I paid - I consider it a few evenings well spent. While I like Apple hardware, the latest round of Apple desktop upgrades was insipid at best, and prices went up about 40% across the board. Perviously I've been willing to pay a premium for premium-quality kit. But not so much that it makes me feel like a chump for having done so.