Well, ive just registered to put foward my experience.
My full System (completely tested and stable) spec is....
ABIT AB9 Pro
E6600 L629F @3.6Ghz, 1.45v Watercooled
XFX 4870x2 with 2Gb Memory Standard cooling
Azuntech Xplosion Sound card
Enermax Revolution 85+ 1250w PSU
3Gb OCZ ATI crossfire certified DDR2 800Mhz (3x1Gb, Stock freq and timings)
OCZ Vertex 250Gb SSD OS drive, 250Gb Western Digital Caviar Data drive
Windows 7 Beta 7068 x64 (also tried in Vista SP1 x86, Same result)
LG DVD/CD RW drive
Anyway, I had a 9800GTX which had 0 problems. I decided to upgrade to the 4870x2, and upon installing the 9.5 Drivers, my system would not boot due to the ATIKMDAG.SYS error that we are all getting.
It booted into safe mode perfectly, and if while in safe mode, i disabled one of the 4870x2 cores in the device manager, the system would boot normally, and function with only one of the GPUs in use.
obviously, i couldn't settle for that. but no matter what i tried, everything suggested on forums all over the place, nothing worked to get both cores going together.
There was however, one strange thing that i noticed. In the system properties of windows, Where before it said i had 3Gb of ram, with the 4870x2 installed, it now said 3Gb (2.7Gb Usable).
At first i thought nothing of it, other than its a bit strange. I had tried the MSConfig trick by this time, and reduced to ram to only 2Gb using that method, but it didnt work.
However, as a last ditch attempt, I removed 1Gb of Ram from my system. (this ram is perfect, there is nothing wrong with it, i know that is what people will say, whatever)
Now, it works perfectly. No errors, no hiccups, nothing.
The only thing i can think of, is that some how, this ATIKMDAG.sys in the 64bit driver package, has some kind of 32bit limitation.
I have tried using a 32bit vista install to test, and i had exactly the same result as 64bit win7.
When adding up all the 'addressed memory' in my system, strangely enough, 3gb ram, 2gb gfx ram, and whatever is on the soundcard... adds up to 5 and a bit... (5.3 perhaps?) Gigabytes of addressed space. now we all know, that 32 bit systems can only address what, 4 and a bit Gigabytes... and oooh look, i took out 1Gb and now it works...
Am i barking up the wrong tree here?
EDIT: i also cant help but notice, that all the people that are having problems, seem to have mor than 4gb of addressed memory space.... amirite?