Well, I don't know that either, So I was assuming people that build computer for a living could tell me.
Although right now, I am just looking to go with the 4870, and advertise it as the best gpu you can get right now. But what worries me is that I dont want to spend over 150 extra on something people will not even notice. But again california is preety high tech, but you never know at who looks at the ads.
For example, today I just sold a computer, and the guy had no clue what anything was, and he believe I was selling cheap (while I was making 250 profit) so he asked a friend that knew computers to test it out. But he never really knew what he got. But again, maybe this was just luck?
But that isn't true. Its one of the fastest cards but is not the fastest card. I wouldn't suggest false advertising. Its a great way to get a bad rep.
To answer the original question...The customer would only notice the difference in performance if they play a game that maxes out the 8800gt.
If their budget is low then the 8800gt is fine but if they want to spend more and get better performance then charge more and explain that its a better card. Let the customer choose.