If the information is correct, even the $399 part will be able to beat the GeForce 8800 GTX:
We don’t know does this apply for all cases but at least in most of the games and benchmarks ATI part should win.
Radeon HD/X2900 XT with 512 MB should end up beating Nvidia after all. We know that ATI has better and faster driver especially in OpenGL and the new driver and 512 bit memory interface will help this card win the race.
From what we know the R600XT with 512 MB might have very similar speed to the R600XTX with DDR4 and 1024 MB but just at cheaper price.
Unfortunately all these claims about the R600 are mainly rumours and chatter. We would like to (finally) see some real results please.