As noted by Tom's Hardware, the R9 285 is based around AMD's Tonga Pro GPU: The board features a clock speed of 918MHz, and a memory speed of 5.5Gbps. Put it all together and you have a board that can deliver up to 3.29 teraFLOPS of computing performance.
The R9 285 is a 190-watt PCI Express 3.0 board, and requires a pair of six-pin power connectors. As Tom's Hardware points out, this compares favorably to the 250-watt power draw of the R9 285's predecessor, the R9 280.
AMD touts the R9 285 as being faster than the Nvidia GTX 760, and says that it "designed it for a single purpose: to play demanding PC games at maximum detail better than any card in its class." We'll have to wait and see if AMD's claims hold up in the real world, though.
The Radeon R9 will be available for purchase on September 2nd, and it'll set you back $249.