You are willing to upgrade the board to gain maybe +10% (peak, average will be closer to +3%) if you're lucky ?
Assuming your SLI bridge is configured for optimal performance the bottleneck will not be PCIe x8 (to each card).
Even just 3 GB/sec (full duplex) to/from each video card in a given system is more than ample, going higher doesn't scale performance very well at all unless running at the highest resolutions over multiple of todays 'prosumer' CRT/TFTs.
i would advice A8N 32 if u willing to use the board for a long time because later nvidia card (high-end) will be optimised on X16 SLI so there will be lots improvement in later games performance...But all it needs is optimisation even SLI doesn't help much in some games....
Other considerations may be the 8 phase power system on the A8N32-SLI. I have this board and really liked what I read about the 8 phase system. It does not sound the the x16 really gets you a large performance boost from what I read, but the 8 phase may get you better stability/cooling performance. I am not into overclocking yet (other than manually setting my memory to 2-3-3-6-1T @ 2.75V). Plan to overclock soon. So far with all stock equipment (listed in my signature) everything runs nice and cool and stable even with the cheap power supply included with my case.
You may want to take a good look at the picture of the A8N32 as well. With 2 GPU's in there I could easily see it getting kind of tight if you have other PCI cards.
Woops... One other thing I forgot to mention to anyone deciding to get the A8N32-SLI is to DISABLE LEGACY USB in BIOS or MEMTEST86 will throw a ton of false errors. I like to mention this lots cuz it pestered me for a week or so.
I'll vouch for things getting tight with two GPUs. I wonder if SLI and Crossfire will really make mobo manufacturers really start to think if their layouts are good enough as more and more upper end cards are using a two-slot design, and how many of the future main-stream cards will utilize 2 slots, and due to the fact that people tend to have at least one PCI card.