Probably use more power, and not be as efficient.
Like the current GTX200s!
It's amazing how something with 936 million transistors can beat something with 1.4 billion, isn't it?
And now 2 Billion is beating 2.8

, while consuming about half the power. GTX295 prices need to drop around $150 to stay competitive, ATI did the exact same thing it did with the HD4870 and undercut the price completely.
Although, the $379 for HD5870 is a bit much for your budget, wait for the $270 HD5850 to come out, then the GTX285 prices will drop like a rock, causing the HD4870 to go into the sub-$100, HD4850X2 will probably drop to $200
Isn't a $270 card beating a $320 card good value?
Nvidia is going to have a big problem soon, like I said, they can't make the GTX260/280 much cheaper, they are going to start losing profit.
On a side note, ATI did have the fastest single GPU before the HD5870, the omegaclock version of the HD4890 at 1.1ghz beats the GTX285

(Does not count dual core GPU's, ala GTX295/HD4850X2/HD4870X2)