ATI is Coca Cola
Nvidia is Pepsi
Fermi is Nvidia's new flavor of Pepsi
the 5970, 5870, 5XXX series is ATI's newest line of Coca Cola products.
Nvidia/Pepsi will be releasing Fermi in a month or so, and it may be a better flavor than the 5XXX series released by ATI/Coca Cola.
So it's best to wait and see if the Pepsi/Nvidia's Fermi is better.
Also, it may push the prices of the Coca Cola/ATI down. (If you're interested in buying a Coca Cola/ATI card.)
That's an analogy I like to think of in my brain.