If the 7900 is 32 pipes and 700MHz, it'd be pretty hard for it not to outpace the X1900, so the issue comes down to price and availability. I'd be pretty surprised to find NV can put out a 700+MHz 32 pipe monster at the same price as ATI's X1900, which seems to be a relatively sensible high-end card (very relative term, but $500 for 512MB and good availability isn't bad).
"Old" as the G70 is, it still kicks ass and trades marketing bullets with R5x0 (e.g., VTF vs. "HDR+AA"). If G71's faster and R580 has nicer IQ, at least we have a choice b/w IHVs, if not settings on a single card.
This thread would worry me WRT future-proofing. Then again, if there's a lesson we've learned time and time again, it's don't get too comfortable with your current resolution and/or video card. And, as 9700P showed us with its shader promise and 6800 with its HDR promise and 7800 with its VTF promise and now X1800 with its branching promise, don't expect too many GPU promises to be filled before the next GPU generation arrives.
I dunno, if G71 is a lot like RSX in PS3, you have to think it'll have some staying power, if only b/c game devs are going to optimize like crazy for it. It's a tougher call with Xenos in the 360, b/c it's probably closer to ATI's next gen than R5x0.
This reply has been brought to you by the word "moot," as the OP appears to have gone with SLIed KOs. Just thought I'd toss some stuff out there.