Performance difference between 6800GT and 7800GS isnt that great. I wouldnt even buy 6800GT at all because it's not worth the price for the performance it gives.
So final word is, money. However, since it doesnt seem like your a hardcore gamer, it'd go for 6800GT. Elder Scroll isn't that GPU demanding so you'll have no problem running 1024x768 @ max settings. Get the 7800GS if u have the money.
Or x800xtpe