Im running Vista 64bit with the Vista 64bit drivers and 2 HD3850s.

In single mode it runs fine but when I enable crossfire I hear the windows ding sound but no second one(normally it dings 2 times and the display comes back) and the screen goes black and stays black...im forced to reset.

When I log back into Vista it does this right away, so then its stuck in failed crossfire mode and I had to go in safe mode and delete my vid card drivers to get the system to come back up...

Is this a possible power problem?

I did install 4(2sticks 2gig) more gigs of ram is that the problem?

hate to ask but what kind of motherboard, power supply and GFX cards do you have?

Gigabyte 790FX motherboard(GA-MA790FX-DS5)
Antec 850watt modular power supply(was checked for stability/wattage output, it passes)
two Diamond ATI HD 3850
Centon Ram -- 8gigs DDR2 800

only other thing I could think of is that I also recently had to revert to the F1 Bios instead of F3B because the computer wouldnt post..

Ive always went with Antec power supplies nevr had a problem with them even though the one I have has gotten real bad reviews at newegg.
Have been going with centon ram for 6years on all types of motherboards even though they are not on the motherboards supported ram list they have always worked.

I also got this BSOD death 2 times now after installing drivers: Attempt to reset the display driver and recover from timeout has failed

Now I have tried a little test, I got 2 HD2600XTs and tried to enable crossfire, same deal but no black screen they just wont enable, it looks like it is and does all the motions and beeps and then I look and the box is unchecked still..