It depends on your preference with chip.
in my opinion, they are just as good as each other, with a slight fps difference on each card, on every test. Sometimes the 7800 wins, and sometimes the ATI version wins.
I would personally go for the 7800GTX Ultra, which is simply the best card out there, comparing, and sometimes bettering the ATI version of their card.
Though, you'll pay a premium for this top gear, but hey, you'll be getting very high framerates, even when running at top settings, in every game out, and probably for a very long time. So theres that factor.
The only bad factor with a card this powerful is that your CPU tends to make a bottleneck for the system, so that the card can only send a certain FPS limit to the screen after it hits a threshold.
Not to worry though, since you have a very fast processor, and it should cope.
So yeah, good look on choosing, but it seems like you go for the best hardware, so I guess it'll have to be a discison based on your part, simply because they are so closely matched. you wouldn't do wrong getting any