Skuzgib - Reserator investigation will be my last resort (It'd be a heavy puppy to return even if the vendor would take it back). On an optimistic note, it's a fairly simple piece of kit and it hums and moves water when it's on : what's more it seems to keep the CPU nice and cool (one web review which trashed the Mobo did concede at least the temps in the BIOS were honest) particularly since I replaced my walnut-whip style thermal paste job. It is one of the more penis-extending aspects of the rig and I think I would rather build a secondary circuit to run through my fridge than replace it with air...but I take your point from a testing/diagnostic point of view. Thanks.
Purple - as part of my RAM tinkerings, I will try your suggestions on dual channel mode. I have however read a very disturbing report on the web that Corsair twins won't run in dual channel on the K8N unless all four slots are full. While I'd like another two sticks, I think I've spent enough for the moment. I could be one of the small minority who were seduced by the 939/K8N mobo reviews and who are now finding compatibility issues with stuff. Perhaps even more worrying is a claim by someone like myself elsewhere on the web that Antec PSUs (amongst others) don't get on well with the K8Ns...sigh.
I imagine my PC (even on non-dual channel) is adequate for HL2 but the more I read to sort my probs out, the more I want to be able post a good "pifast" result or boast about how many hours of prime95 my machine withstood. Arggghh. Must get it to work.
Keep writing.