It depends on how much you have to spend.....if money is not an issue then go for the GTX 280. If you don't have a huge bank account then go for the 4870.
Look at some benchmarks.....the GTX 280 walks on the 4870! The 4870 does well at higher resolutions, but overall, the GTX 280 is the single fastest card made.
This is from MaximumPC, they did a head to head benchmark test with the same system, just changed drivers from manufacturer website....the numbers don't lie
..................................4870.........4870 in CF........GTX 280
Crysis..........................9.3fps..........19 .9fps............15.9fps
World in Conflict............28.0fps........34.0fps........ .....32fps
3Dmark06.....................34.0fps........47.1fp s.............46fps
3Dmark Vantage G1........10.7fps........19.9fps............15.5fp s
3Dmark Vantage G2........9.0fps..........17.4fps............11.9f ps
I am not saying that the 4870 is not a good card.....It is a great card for the price

I have actually considered buying one. I am still nervous because all I have used my entire life is Nvidia GPUs. Right now I have an Evga 9800 GTX and the 4870 will crush it!
I would bet that in about 30 days the price of the GTX 280 will drop into the mid $300 range. Nvidia knows that people are buying the ATI cards because of price

erformance ratio and they are loosing money because of it.
The GTX 260 and the 4870 are almost the same performance, you can get a GTX 260 right now for $250 That is about the exact same price as the 4870.
EVGA GTX 260 Superclocked Edition for $250
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130372