Originally Posted by lemniscate
umm, I think I said that I'm going to use slot 1 and 4? didn't even think about putting one in slot 5...
I also considered using slot 1 and 6, then put the sound card at slot 4 or 5, but that would block most connectors at that end of the board (USB, molex, etc.), so I have to check whether they'd fit or not first (especially the molex, since the connector is vertical, not 90 degree angled).
so my question is still unanswered: should I go for the windforce or the reference?
To be clear, what karlitos is stating is that you cannot simply pick and choose which PCI-express slots to use for 2 way SLI - if you do, they will run at the incorrect speeds (eg PCI express x4). Your motherboard manual will explicitly state WHICH TWO SLOTS you are *required* to use for 2-way SLI, as it does in the picture shown by Karlitos. You can't simply shift them around at your leisure unless you want to run them at sub-optimial pci express speeds.
I've owned many SLI asus and gigabyte motherboards and the manuals always specify which two slots to use for 2-way SLI. Again, you can't pick and choose. You need to use what is specified in your manual. Therefore you can't use "slots 1 and 6".
To answer your question: reference is easier and less hassle for SLI, while aftermarket cards require some tweaking and attention to air flow for SLI. For single card, it doesn't matter.