Good luck with your new build. Let me know how you like your A8N- SLI.. . Which PSU are you planning to use? These SLI rigs seem to be needing plenty of power on the 12V- they are recommending at least 20 AMPs on the rail for the more powerful GPU's, and dual rail might be the best way to go. Otherwise, why are manufacturers like Seasonic coming out with SLI PSU's with dual 12V?
I was also looking at the Freezer 64 but decided against it because of the fact that ~1kg worth of copper should be able to cool the winchester cpu pretty well. In theory anyway.
Powerwise I'm going for a Tagan 480w. I was considering a whole lot of different psu's, including the Thermaltake 680w but after reading more or less the whole A8N-SLI thread over at anandtech there seems to be a problem with this card and dual railed psus. I have no idea if this would apply to all dual/multi railed psus or just some, but a lot of people over at anandtech who had stability issues resolved these with a new, single rail psu. This doesnt seem logic to me at all, but I figured I'd save myself some headaches and just go for the Tagan. Another reason for the Tagan 480w was a forum post over at another forum by the site admin who had actually run 6800 Ultras in SLI powered by a Tagan 480w.
In other words, it's known to work with a setup that exceeds mine and thats allways a good thing. I was also considering the ocz powerstream 520w but decided against that as well simply because of the "known good" config with a tagan. Tagan are supposidly produced by Topower (.....) which is never a good sign, but they've gotten good reviews around the net. Toms Hardware also had a Tagan PSU die during their AMD / Intel stabilty shootout, but I took the chance anyway.
As of the why's of the dual rails.. It's a nice idea, I think the problem people are reporting is more related to a unusual high power draw on startup, this might be motherboard related and not a bad thing about dual rails. However, I'm quite sure the ATX specs where designed with other things in mind; drawing 20+ amps of 12v is, in my opinion, quite insane. We're almost talking the levels you'll find in a car here (not quite but still..!).