Our colleagues over at NordicHardware have been trying to get a pair of Radeon HD 2900 XT's in Crossfire to overclock for some time, and I am sure that they are not the only ones.
They managed it with some help from AMD and they got the GPU to work at 880 MHz with the memory overclocked to 950 MHz, and they even tweaked the memory latency. With these clocks they managed to get 20,278 points in 3DMark 06. The system used to benchmark the cards was an Intel Core 2 Extreme QX6700 running at 3,762MHz on air cooling. The motherboard was an ASUS P5W DH Deluxe and the memory was a pair of Corsair Dominators at 456MHz (3-3-3-5).
"With a completely air-cooled system Kinc managed to score 20,278 points in 3DMark 06. Both the SM2.0 and SM3.0 scores are very good for such a low CPU frequency, 7,989 and 9,449 respectively. We can also tell you that CrossFire scales well, really, really well."
The last sentence is my favorite "Don't worry, liquid nitrogen results are coming...". You can check out bigger desktop screenshot over at Nordichardware.