They won't be both. They're not stupid, so if they can't compete with the speed of the 5800s (and nVidia are already sounding confident that they can beat the ATI cards, though I'm not sure how much they can be believed), then they'll have to compete on price by making their cards cheaper, and ATI will surely follow suit.
If you are looking for a card right here and right now yes it's a well worth buy. Like said earlier drivers are still somewhat new and immature and greater performance increases will be available shortly. If you could wait a while more why not save up for a 5870? It's a beast. and NVIDIA is going to be overpriced again this time around thanx to their physx ageia branding and the overhyped brand name.
I'll believe them when I see benches. They can be alluding that they'll beat the ati cars in price for all we know.
And two other things: EyeFinity (which, admittedly, isn't for everyone) Vastly superior load and idle power draw.