Marcus Kinc Hultin has overclocked both X1950XTX and X1900XTX and set a new single-GPU world record with the latter.
"An ASUS-made Radeon X1900XTX graphics card got its cooler exchanged for a
two-stage compressor cooler which forced it down to -80°C and moved the
frequencies up to a whopping 909MHz GPU and 1800MHz memory. Intel's X6800
processor was equipped with a similar two-stage setup with temperatures
down to -100°C and this resulted in a frequency of 4.69GHz using a 11x
multiplier and a 1706MHz processor bus."