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Most of my use is under Windows/Bootcamp, so for a quick test, I timed how long the 5870 and the 7970 took to thumbnail ~150 home movie files (720p/1080p AVC HD files) when opening the folder in Windows Explorer - both setups took the exact same time..

Windows makes no use of the video card to generate thumbnails so this is no surprise... the bottleneck here will be your disk and/or CPU.
 
7870 Eyefinity 6? It has 6 MDP ports and uses 2x6 power connectors in a single card. I have no idea if it works in OS X though.

Update, went with the 7870 eyefinity and Windows stable/perfect, thanks for heads up no clue that this card even existed. I understand now re: thumbnails and wasn't the best quick test I could do but now having used it for a few weeks when editing/viewing media, I no longer see any lags whereas the 5870 would take time to process plus now have any easy way to add more ACDs.

I'm just waiting for the 10.8.3 release so I can run it on the Mac side as well.

Note, though, both the MSI 7970 and these 7870 eyefinity cards can only be used in the 2nd x16 slot. They both have these protruding items on the backplate that prevents the card from sitting flush in slot 1.
 
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