Oh, I didn't say that 650i was the best way to go, I was just pointing out that there were more options than just 680i...
I think that keysplayer2001 has the Asus P5N-E SLI 650i board, and I believe he's quite happy with it. The drawback to 650i is that that boards generally aren't as feature rich as 680i boards, and SLI is 8x/8x instead of 16x/16x. On the flipside, I talked to a guy at Fry's recently (who actually seemed to know what he was talking about), and he said that the return rate for the Asus P5N-E SLI was much, much lower than the return rate for the EVGA 680i SLI board.
I personally had one of the BFGTech 680i boards (reference design like the EVGA) last fall, and I returned it because I couldn't get XP to install without encountering a BSOD. Apparently, the reference boards no longer have the issue that caused the issue I experienced, but after spending $300 (at the time) on a board with issues I decided to give 680i a miss. At that time, I just choose to run my Core2Duo at stock speeds and stick with my nForce4 SLI 16x Asus P5N SLI SE Deluxe, which was a super board at stock speeds. It doesn't support quad core, or I would highly recommend one for your setup.
If I was in your shoes, I would continue to run your AMD rig until the 9-series cards are launched to see if NVIDIA also launches a new chipset. 680i is getting a bit 'old', and I can't image that NVIDIA is going to launch the PCIe 2.0 9-series cards and not also introduce a new PCIe 2.0 SLI chipset. Also, Barcelona benchmarks should be out and about soonish, which will give us some sort of indication of how well AMD's new Phenom processors will perform. Intel quad-core may not be the fastest chip on the block for too much longer... Then again, maybe it will... Either way you slice it, now is not the best time to build an SLI rig IMO.
If you absolutely have to get a Q6600 and an SLI mobo, I'd suggest getting a relatively inexpensive 650i SLI board with the intention of replacing it with a new SLI chipset board in a few months. My wife is going to inherit my P35/C2D rig (overclock and all), and I'm going to probably build a 9-series SLI/quad core rig when the new stuff becomes available. I'm undecided as to what platform and/or chipset I'm going to opt for though. If Phenom performs close or better to Core2 Quad, I'll opt for whichever platform has better SLI support. Likewise, if on the off chance the new Intel X38 chipset ends up with SLI support, it will definitely be up for consideration. It's just a bit tough to say at the moment what is going to be the best way to go.