I think your board has a PCI-e x16 and a PCI-e x4 slot on it, so crossfire might work but I don't think the motherboard supports it.
To run 3 monitors with nVidia 3D surround you need 2 or more cards. With ATi's Eyefinity you only need 1 card.
I would say you can't go far wrong with getting a HD5870, BUT there should be a new set of 69 series cards coming out just about now, so if you can wait a little while you can expect better performance.
I would also just check your power supply, not that the 5870 uses much more power, but that is normally the problem a lot of beefier systems have.
Hope to of helped!
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