Why not wait for R600? True, it's been pushed back, but given the driver issues under Vista that nVidia's run into (but, then again, they've had barely any time to work with the final version of Vista before it's release).
The P5W-DH can still run the fastest Core 2 Duo's out there, so I don't see the motherboard as being an issue. ATi has very good hardware, and with AMD cracking the whip, I'd expect the drivers to be stronger and more timely than before the acquisition.
As far as motherboards go, the reference design 680i boards are the same thing with the company slapping it's name on the motherboard and selling it. Asus and Abit are the only companies I've seen with a different design on it, and they've had a few problems...well the Asus model. I just don't see why you'd pay more for a motherboard that's got a few extra frills and costs at least $100 (sometimes $200) more than the competition.
However, in this case I agree with Izzy: you've got a very solid system as it is, and really, I'm perplexed as why you'd want to get rid of something that good so soon. Next year, nVidia and AMD (ATi) will have something else out that's bigger and better. If Intel changes the VRM spec for Penryn (Conroe successor), you'll probably need a new motherboard then. If you still want to upgrade, keep the motherboard that you have unless you're into extreme overclocking...and even then, I'd have second thoughts. The Core 2 X6800 is the fastest processor available right now.
If you do go ahead with the upgrade, make sure you have a solid power supply. I don't know what R600 will be like, but the 8800GTX's require about 300W apiece, so take that into account.