PCI Express is a serial based technology. Which means data can be transfered in both directions. PCIe 16x is capable of up to 4000MBps transfer rate in both directions. This is true of one single card. Up to this point most SLI motherboards that have (2) PCIe 16X slots have only been able to use 8x bandwidth or roughly 2133MBps for each slot. Which is also roughly the same transfer rate of the current AGP slot. So you're taking a card & serial bus slot with (16x) capability and limiting it to 8X bandwidth. Although the combination of two video cards does increase performance, this current SLI technology with (2) 8x bandwidth is limiting the PCIe technology.
Enter SLI motherboards that support 16x & 16x = 32x Bandwidth.
Which claims to provide up to 8000MBps bandwidth potential.
Well how does this potential stack up against the current 4000MBps available?
Here is an article from HotHardware.com where they've basically reviewed, benchmarked and provided conclusions to this new technology.
If you don't want to read the article i'll sum it up for you.
They basically discovered that this new technology only provides a must noticable increase in performance when running games like "FRYCRY" at Highest Resolutions with 8X SLI-AA - 16xAF vs. 16x SLI-AA - 16xAF between the 16x boards & 32x boards.
Which IMO for an Extremely High-end gamer, this technology is a no brainer. If you're going to spend $5000.00 on a gaming beast, you might as well get the absolute most out of your system.
As for the ASUS A8N32 motherboard, they need to redesign this board to better support use of it's PCI slots. People whom want to use high-end Soundcards or other PCI cards will be at a loss for real estate and take a hit in the thermal department. This board just isn't layed out properly.