GTX 670 2-Way SLI on PCI-E 2.0 x8?

[PetrolHead247]

I've been sitting on this thought for a little while now, and i was wondering if you guys could help me out. I've been thinking about getting 2 Asus GTX670's at the end of August and SLI'ing them, fair enough i hear you say. However, i have a 2500K which i plan to keep and take out of my Asus P8P67 and put it into something like an MSI Z77 GD65, Gigabyte UD5 or maybe the MSI Z77 MPower if i will be able to afford it, but because the CPU can only run the PCI-E Lanes at 2.0 mode, two 670's would have to run at 2.0 x8 speeds (3.0 x4 speeds).

I was wondering if having 2 670's running at 2.0 x8/3.0 x4 would be a bottleneck to the cards? I know that PCI-E 3.0 cards running in 3 or 4 way SLI on PCI-E 2.0 take quite a large performance hit, so i'm wondering if this is the same for 2 way SLI or not. I've looked around teh internet for a while but i can't find anything solid on SLI'ing 2 Nvidia 3.0 cards in 2.0 mode, just stuff on 4 way SLI and single cards. Another option is of course just going with 1 670, but the SLI performance from what i've seen is awesome.

 

[Master&Puppet]

From what I've read around on I'd say that you could potentially see up to around a 15% fps drop from the true maximum scaling by running it that way. It depends on the game. I haven't actually seen anything GTX670 specific so my thoughts are based on this review of a 7970:

http://www.anandtech.com/show/5458/the-radeon-hd-7970-reprise-pcie-bandwidth-overclocking-and-msaa

In any case the overall fps you'll gain will easily outway the potential bottleneck in my opinion. Some people will say that 2x 670s are overkill for 1 screen but I have 2x7950s which are similar in performance and I don't feel that I've wasted my money - instead everything is butter smooth.

It is certainly not worth upgrading your cpu and mobo for unless you've got an issue. I'd just wait until you know the platform is holding FPS back (either by having too weak a CPU or by slightly gimping SLI performance). It will probably be a year or two before you feel either of those situations arriving, second gen haswell I'd imagine.

There's not much else I can add other than offering to try out the difference between pcie-3 and 2 on my own pc for you to get some more figures if that helps. It would of course be on a pair of 7950s though.