At this price point it's mainly between:
1. The Geforce GTX 470 ($245+)
2. The Radeon HD 6870 ($250+)
Overall, the GTX 470 has a very slight advantage performance wise, but that comes with three major tradeoffs, compared to the Radeon HD 6870, namely power consumption, temperatures and noise. The 470 uses a whopping 100-110W more than the 6870 does at load. It also is roughly 3 times louder than the 6870.
Although I'm a Nvidia man, I'd personally suggest the 6870.