Crosshair III / Memory POST problem

After having spent the last ~24 hours searching for a solution to the problem I'm having I have to throw in the towel.

I am currently building 2 new rigs to replace my own and my girlfriends with teh following specs;

Chassis: Thermaltake Tsunami VA3000SWA
PSU: Corsair TX750W
MB: Crosshair III
CPU: Phenom II X4 965 BE
Graphics: Sapphire Radeon HD 4850 1gb crossfired
RAM: 2 x 2 gb Crucial 1066 mhz ram (CT2KIT25664BA1067)
HD: Western Digital Caviar Black, 1 TB (WD1002FAEX)
Optical drive from the old computers.

I am struggling to get the first rig I completed to POST regularly, the LCD poster hangs on DET DRAM as sson as I have both sticsk of ram in, and when I have only 1 stick in in any other slot than the A1 channel. it has POST'ed succesfully and almost consistently with only 1 stick (and with all 4 sticks I have available - 2 for each build) in the A1 socket.
I have tried the MemOK! function for 2 sticks in A1+B1 (as per the MoBo Manual p. 2-15) and left it to run. After haing gone through all th etunings (I assume) it stuck on DET DRAM again.
I have gone through the "checklist" superficially - superficially because I "know" its the ram thats the culprit somehow (thanks to the LCD Poster).
From the research I've done I think I have to manually set the timings in the BIOS with 1 stick in to get to BIOS, but when I do that (7-7-7-10) it still hangs on DET DRAM when I put in the second stick.
I have tried having the 2 sticks in A1/B2 to see if it would post as 2 singlechannel sticks, but no luck there either.

The second computer is, as yet, unbuilt, but will build it today (identical specs).

Can anyone here help me with figuring out either the timing settings I need to use, or if I've got fixated on a wrong issue here.