I've never seen a geforce 9800. Do you mean ati 9800(or 9800pro), or a nvidia geforce 6800 (or 6800ultra/gt, etc)?
Anyway, the answer to your question probably is that the "powerhouse cards" like nvida 6800gt's can run at lower clock speeds than the 6600gt because they have more pixel pipelines. I think a 6800gt has 16 pixel pipes and a 6600gt has 8. So a 6800gt doesn't need as fast of a gpu core to do more work than a 6600gt.
Maybe an example will help?
Ok, you got one road, the "6800gt" road. It has 16 lanes going 350mph. If there was one car in each lane in one hour they would have traveled 5600 miles all togther.
Another road, the "6600gt" road has 8 lanes going 500 mph. So if each lane had one car, in an hour they would have gone 4000 miles together.
So even with the 6800gt's lower clock(350) it still outpaces the 6600gt's high clock speed(500) due to it's more pixel pipes.
That maybe a bad illustration, but I think it shows the idea.
I was talking about a nvidia geforce 6800. That was a great explanation thanks a lot but I was looking at newegg and saw this card http://www.newegg.com/Product/Product.asp?Item=N82E16814150106
newegg says that it is a geforce 6800 GT but I think that newegg made a mistake calling it that, insn't it just a "regular" nvidia geforce 6800? I was just wondering what you guys thought.
I doubt that xfx is really a 6800gt. I mean it's $100 less than all the other 6800gt's. It looks physically like a 6600gt, and is also priced like one. It doesn't say the core clock, but it has a 256bit memory interface, and a 700mhz memory clock, which would make me think it's a 6800. I don't know, that seems too cheap for a 6800 or 6800gt, and no reviews yet to find out.