My motherboard a Gigabyte UD3H-B3 Z68X is a PCI-Express 2.0 motherboard now my question isn't about performance difference between PCI-Express 3.0 and 2.0 because I know right now it's really nothing in terms of gaming performance. But when I decide on SLi'ing 2 GTX 670s on my motherboard and my motherboard does sli 8x/8x and nto 16x/16x. Will there be a big difference in performance? Thanks ahead of time.
High-end cards will receive a slight performance hit. Mid-range cards a little lower, and low end cards will not see a difference. For your GTX 670, you will see probably a tiny bit of lower performance (probably 1-2%) on a PCI-E 2.0 x8 slot.