UPDATED 2-2-2013 1:30PM EST (see bottom)
PROBLEM; Could not get HD 5670 Radeon to work with onboard video.
SOLUTION: YES, you can.
FIXES: My stubborn BSOD's (7B and 101) & Spanning desktop across monitors (7200x4320 total area)
I am running an ASUS EVO board with an onboard Radeon 4200 and a Radeon 5670 in the PCIex16 slot. Had ALL kinds of problems until I did about two days of testing and research. I connected six monitors to all the ports available on the machine (three for onboard and three for HD5670 card) to see when the popped up with video based on settings ... I would always know what port was sending video out withoug switching cables around.
After trying to install drivers from just MS and using MS updates, then rolling these back to restore points, and playiing with just AMD drivers and rolling back (having to actually boot to a third drive to manually delete things on the final drive), I met with success (notes below give some other settings I had to make).
PROCESS I USED:
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1) Power plug out of course, discharge fingers, & Install AMD Radeon 5670 Card in machine
2) Power up & tell bios to use onboard graphics FIRST
3) Boot into Win7 64bit
4) Install CCC version 13.1 from AMD site, latest driver available.
5) Disable HD5670 then Reboot (remember, running off onboard video)
6) Install CCC version 13.1 from AMD, LEGACY drivers for 4200 onboard
7) CHECK the CROSSFIRE OPTION
8) Reboot
9) Win7 64bit Ultimate loads up, enables the HD5670 automatically
10) FOUR MONITORS active, two off onboard and two off HD5670 card.
--> NOTE, using the VGA & HDMI from the onboard
DRAWBACKS:
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1) Windows turned off some visual styles, but I don't care about that, I need REAL ESTATE BABY!
2) Video is not as snappy with four monitors attached, dropping to three and will keep tweeking
3) Lost some in my Windoews Experience Index ... went from 7.5 to 6.1 so tweeking is needed
NOTES:
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I ALSO DISABLED ACHI in the bios on the SATA drives and went to IDE. This helped the system to accomplish the above there was some clash there. I have updated the registry to allow for ACHI and enable the drivers in Win7-64 but am not working with that until I get it tweeked. SO ... IF ACHI was part of the problem, or you can't get the above to work, enable IDE in the SATA settings in BIOS and try that.
ULIMATELY, YES YOU CAN HAVE FOUR MONITORS ... IT DOES work spanning the desktop with onboard AND PCIe card ... but you have to have the same brand, can't mix them up!
Will check in later if it explodes! Can load all specs & BIOS settings if anyone needs it, won't waste time if not needed :~)
Cheers.
Mobo: ASUS M4A785TD-V EVO
RAM: 16GB 1333
PSU: 600W
CPU: AMD Phenom 6core (WITH ACC enabled in BIOS)
Other Cards: PCI RealTek 1GB, Hauppage 1800 Tuner Card
Drives: Not that it matters, but running multiple SSD drives (OS:256G, APPS:125G & SWAP:40G) and a couple 1TB SATA drives.
Performance Note: After updating as above, CPU/MB temps went down almost 10C, running much smoother after process above.
UPDATES
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2-2-2013 : CPU still running cooler, all three monitors still working perfectly. Somehow my RAM got bumped while I was in the machine and realized I was only reading 8GB RAM, screens were jittery. I reseated the RAM (remove and replace until they clicked into place) and now showing 16GB the system is very smooth, AREO runs great, and a single picture (6K x 2K picture) spans all three deskops, very nice. So yes, this can be done and it continues to work great!