At 1GHz, the ATI Radeon HD 4890 is the fastest-clocked graphics card in the planet—a world's first. It's not a new chip, however, just a "factory-overclocked" air-cooled 4890 that looks like a Ferrari.
So that's a bit of a cheat: In theory, you can buy the 850MHz version and overclock it yourself without paying the premium for the 1GHz version. Or maybe you can buy the factory-overclocked 1GHz version and overclock it again so your PC melts and reaches the Earth's core. [Hot Hardware]