Radeon X1800XT works at 625MHz /1500 MHz
 

BACK IN August, ATI told its partners that 600/1400 are the clock speeds that it was aiming for, but it didn't tell them that those clocks would be the final clocks.
Around the same time, we received news that R520 - now officially known as the Radeon X1800XT - is going to be overclockable to skyhigh limits. We still stick with our story that the cards are going to work at 700MHz+ but this of course won't be the shipping speed. Yields would probably be very low at those speeds.

But if you look at the memory ATI is using for chips that you can clock to 1600 MHz, there's still a lot of headroom there. We suggested that memory will be very overclockable and we haven’t been wrong on this one either.

It turns out that the new speeds ATI is touting secretly to its partners for the R520XT are 625MHz for core and 1500MHz for the memory. I believe that this is going to be just enough to beat Nvidia, which has a secret weapon called Detonator 81. I think that ATI's lab boys are testing a new driver as we speak, and at this time there is absolutely no way to say that even 625MHz/1500MHz will be the final shipping speeds. It's all on ice, and ATI also wants to throw sand in its competitor's face, no doubt.

One more thing. Whichever wins, the Geforce 7800GTX or Radeon X1800XT, it's going to be by a very small margin so if you own Geforce 7800GTX you should be fine.

http://theinquirer.net/?article=26378

in the "review" done by Hardware Analysis the difference was very small also.. :)