Rabbidrabbit: Did you try going into the bios and selecting single card ? (not Auto). I was pulling my hair out with the same thing. I run an sli system also with the same mobo and XP Pro 64 bit and was having no luck with Aces High on sli even though I had loaded a profile with single gpu selected. It would hard lock within a minute or so after starting the game, sometimes immediately. As soon as I selected single card in the bios ( I had tried both the dual card and auto settings ) all lockup problems went away. I run dual 7800 GTX's but that shouldn't make a difference over the 6800. Let me know if I can help further.
Ted Scafe
After I posted this I decided to try "auto" in the bios one more time , started Aces High and it locked up within 30 seconds. Back into the bios, selected "Single Card" ... lockups gone. Now running 4X AA, 16X AF, 1600/1200, 1024 textures and a getting a solid 75 fps. I disabled the reflective water though as that seemed to drag the frames down. Hope this helps.