The Intel P965-powered Gigabyte 965P-DS3P was only able to use a single GeForce 8800 GTX as it doesn't support SLI. Though, as you can see from the graphs, SLI does not always guarantee more performance.
To be fair with Nvidia, we must also say that a single GeForce 8800 GTX is usually powerful enough for running any of today's most demanding games with your nice share eyecandy.
The P5N-E SLI does exceptionally well using two GeForce 8800 GTX cards and the PCIe 8x slots clearly have no impact on performance, at least not a negative one. There is no difference in performance at 1600x1200 between the P5N-E SLI and its bigger 680i SLI brothers.