I recently added an 8500GT so I can support a third display. 8800GTX is powering Monitors 1 & 2, and 8500GT is powering mon 3. It was fine for about a week and then I started to get the IRQL_NON_LESS_OR_EQUAL errors after less than 1hr of use. This is using an Asus P5k-premium which supports Crossfire. I'm not trying to crossfire it or anything. I just want to run the 8500GT in PCI-e x1 mode for the 3rd monitor without any issues.  
 
I checked for IRQ conflicts and didn't find any for the GPUs or any other devices.
 
I tried using WQHL drivers and the newest beta drivers and it still gives me the BSOD. Any ideas? Or has anyone successfully run two nvidia cards on crossfire capable board but without enabling crossfire?

Uhm, dude, crossfire is for ATI cards, not nvidia.  If you are only running a single card, it's cool.  That's probably why you are getting problems.  Now when I did multimedia at Church, we had a converter box, it was basically you used your 2nd monitor's hookup and plugged it into an input on the box, then it had like 4-8 outputs and you could run however many you needed, and it treated it all like your second monitor.  So maybe something like that.  But I'm betting the board is not liking the fact that your trying to run 2 cards both nvidia in a crossfire board, and your not trying to do crossfire, the board I'm certain was not made to run that way, that's probably why it's going insane on you.  

@OP: "It was fine for about a week" - so, you actually got all 3 monitors to work, no problems? That proves that the Crossfire feature of the board is not the problem. Something must have happened after that week. Try to remember what you did around that time. Did you install or uninstall any software, any drivers, any Windows updates? Did you change some Windows services from Automatic to Manual or Disabled? Did you overclock one of the video cards? Run memtest86 too, maybe your RAM is starting to fail.