This weekend there was a ATI R520 party in Ibiza and VR-Zone says it was pretty chilly over there..
ATI had invited world's best overclockers and asked them to threat the Radeon X1800XT with some LN2.
It looks like the card reached a core speed of 856MHz with 1880MHz memory and probably the memory is even capable of passing the 1900MHz mark.
The 3DMark score of this overclock isn't know but it's probably 11,200 or more. Check it out over here.