As long as it's got 8 pipes (not a 9800SE) and 256-bit memory, they're all pretty much the same in terms of performance. ATI offers a 3-year warranty on "their" cards; most others are 1-year. And if you buy a retail card you usually get some (really lame) games with it.
Also, some of the more expensive cards have better cooling solutions, or come overclocked from the standard speeds (380/340). You'd have to look at the details on the individual cards, though.