2 -4% is the loss on 8x versus 16x with lower power cards (HD58xx series) than GTX580 (might be a little more there). At your resolution though it is going to be a while before you will have to upgrade to the SLI setup.
OK, so i've got some money saved up right now and am wondering about a few purchases.
A. I could buy a 2600k with a P67 board.
B. Wait for LGA 2011..
Also, would a 2600k bottleneck a GTX 580 sli setup? Seeing as I've already got one and will be buying another when one is not enough for the latest games to show up later on.
I see you've already selected a "best answer", but here's more food for thought:
1.) ATI cards (quoted in a response) are more bandwidth-dependant than Nvidia cards, so you'd likely see "minimal" losses going to x8/x8
2.) The NF200 PCIe bridge has a repeater function that basically sends the same data to both cards simultaneously. Nvidia calls it "Broadcast", and it's a complete equalizer when it comes to SLI since both cards in the array need the same data.
3.) If you're worried about bandwidth, NF200-equipped LGA-1155 motherboards are already on the market.
As soon as 2011 arrives something better will be in development. Waiting is a waste of time.
disagree with you at the moment!
I would recommend to OP to wait for LGA 2011, cause it will bring PCI-E 3.0 and 4chanel memory
PCI-E wont change so quick, but if you will go now for LGA 1155 you will stack with still the same PCI-E 2.