AMD's cards have always been known for better power consumption and efficiency than NVIDIA cards but in this case the all three GPUs are very comparable. The HD 4850 does use about 16 watts less power than the 9800 GTX and 18 watts less at load - enough to warrant a 6-8% advantage over the G92.
Performance, Pricing and Availability
The AMD Radeon HD 4850 512MB card is a good performing card that is able to match both the GeForce 8800 GT and the 9800 GTX card in our initial preview of just a couple of major gaming titles. NVIDIA has some more tricks up their sleeve though with a little part called the GeForce 9800 GTX+ meant solely to rain on AMD's launch day parade. Whether or not that will pan out has yet to be seen.
The estimated MSRP on the HD 4850 512MB cards is $199 - low enough to be competitive in the market against NVIDIA's solutions. Currently the GeForce 9800 GTX is selling for no less than $269 while the 8800 GT cards are going for anywhere between $159 and $209.
Availability of the HD 4850 should be immediate and no later than June 25th - you'll have to wait a couple weeks longer to get your hands on the HD 4870 graphics cards