The only problem with the ASUS P5N-E SLI is it only has one PCIe 1x slot available, with other 1x lane being dedicated to the on-board Gigabit LAN controller.
Another flaw is that the SLI graphics cards are forced to share 16 PCIe lanes, so when SLI is enabled the bus is divided effectively creating dual PCIe 8x slots.
Now, the old AGP 8x bus peaked at 2.1GB/s and proved to deliver more than enough bandwidth for the latest graphics cards simultaneously. Then there is the PCI Express 8x which can transfer up to 2GB/s in any given direction simultaneously (allowing for a potential bandwidth of 4GB/s).
To be Continued…