Yes, they can, but that's going to restrict performance if this is a high-end GPU we're talking about, since PCI-E 2.0 x4 = PCI-E 1.1 x8.
I have a GTX 670 FTW so, I'm going to SLI it. Since I need a new PSU, and GPU buying a motherboard would be really expensive. So if I can get a second one and at least see some performance increase until I get a new motherboard I will be happy. So if I SLIed on the Motherboard http://www.newegg.com/Product/Product.aspx?Item=N82E16813130699
what kind of performance increase would I see? If you get about 180-200% normal increase, would I see around 120-150% increase?
Thanks for the help!