SLI 460 Question

I already have a single graphics card and i want to know if i bought another gtx 460 for SLI would i see an increase that would be better than spending a little bit more for a 560 ti.

I would obviously go with the 460 for SLI, but my mobo GIGABYTE GA-P55-USB3 and the pcie speeds make me wonder if spending like $40 more for a 560 would out do the performance of sli 460s with x16/x4 speeds. Upgrading my mobo is not an option right now.