Both are sufficient. The 7800XT is barely faster than the 6800XT, it could almost be considered negligible. If given the choice (all else being equal), I would go with the 7800XT, just for the fact that it's newer and therefore comes with a slightly better feature set.

~~That being said, the 7800XT has worse energy efficiency than the 6800XT. I don't remember the numbers off the top of my head, but if you care about either energy efficiency or heat output, I would consider looking into that before you make a decision.~~

~~I've heard that you can knock off like 50% of the energy usage for 20% less performance from the 7800XT, but then that brings up the issue that if you're willing to do some performance tweaking on the 7800XT, you're likely also willing to do performance tweaking on the 6800XT. So, your mileage may vary on that.~~

Edit: I misremembered - 7800XT is more power efficient than 6800XT. There should be no reason to go for the 6800XT if they're the same price