Newbie Crossfire Setup

I've built and overclocked computers since BC (before Clinton), but so far setting up my first crossfire has not been successful.

The ingredients:
__ Corsair 750Watt PS
__ 2 Asus 4850 video cards (VCs)
__ Asus P5Q Pro MB w Q6550 CPU (running stock speed during this attempt)
__ ATI Catalyst and drivers downloaded yesterday
__ Windows Vista Premium 64
__ 4GB Patriot DDR2 1066 ram
__ 1 sata HD, 1 CD/DVD

The symptoms:
* Both 4850s work fine as the only VC
* DVI cable plugged into lower slot on primary PCI-e card
* When I install two 4850s, I sometimes get an error 43 in Device manager for one of the 4850s, but sometimes I get both VCs recognized without any errors
* CCC only reports one VC. When I disconnect the two Cross-fire bridges between the VCs and start up CCC, I get the crossfire section available but of course it does not offer the checkbox for crossfire because the two VCs are not connected
* I shut down, connect both crossfire bridges, start up, and towards the end of the Windows Vista 64 load I get a BSOD.
* I could not find any cross-fire-related items to change in the MB BIOS

The alternative:
I have a third video card, a 4850 with 1GB of DDR3 (the other two have 512MB). When I substitute the 1GB card for either of the others I get the same problems

Since I am a newbie at cross-fire, you won't hurt my feelings by asking obvious questions.
Papafox

Maybe follow this guide http://forums.techpowerup.com/showpost.php?p=635797&postcount=1

OnDborder,
Thanks for the excellent link. I followed the suggestions and managed to get crossfire working. Looks like updating my Vista files was the solution. Running 3DMark06 and see a substantial improvement, and everything is okay in CCC, too.
cheers,
Papafox

ps: I overclocked cpu to 3.8Ghz and got a 3DMark06 score of over 20,200. Not bad for a couple of $170 video cards