PCI Express, SLI questions

I am currently in the process of building a new system, I just am stuck on a few choices. I currently have a PCI Express geforce 7600 gt oc from BFG. My questions:

Is SLI worth consideration (2x geforce 7600 gts), meaning nforce 590 chipset.

And secondly, one of the boards I was considering with SLI support, when running only 1 pci-express card, the slot only runs at 8x. How much of a performance decrease is this as compared to a x16 pci-express slot? (card runs at 16x)

I have been reading up on this for the past week, and still cant come to a conclusion.
Thanks