They can both do HDR and they're both awesome. But the X1800XT is better than the 7800GTX. However, the problem comes in when you consider the prices and temperatures. I see that you linked the $569 7800GTX, here's the cheapest reliable one;
The X1800XT IS NOT $100 better than the 7800GTX...
Also, the X1800XT runs A LOT hotter than the 7800GTX. You know how you're supposed to stop overclocking when a card or processor gets too hot? Well, the X1800XT comes "too hot" at stock. It has amazing overclocking potential, but nothing that an average PC user will ever be able to achieve with air cooling.
My vote? I like Nvidia, so 7800GTX. Better drivers, better hardware.